Hello, Everyone. I have been following this thread for years but finally decided to post some possibly inciteful observations on 2 pair of Bose 601's I've come across... the series II & series III. (And later, the 501 Series IV)
I first came across a set of 601 series III a few years back when a guy wanted me to do some repair work on his vehicle at his house. When I walked up to his front door & knocked, I noticed as he opened up the door a pair of Bose 601's inside (in decent shape cosmetically). As I began repairing his vehicle, I asked him about the speakers. He said they were "just some old speakers and weren't even plugged into the stereo". I offered to do the work in trade for the speakers & he jumped at the offer without hesitation. (If he had hooked them up, I probably would not have them now! ) I took them home later & noticed that all 4 woofers needed to be refoamed. The cabinets had REAL wood veneer that needed cleaning & a light sanding with superfine sandpaper plus an application of Howard's Feed & Wax. Other than that... perfect.
A few months ago, I walked into a local thrift shop & walked up to a MINT pair of 601 series II. The ONLY problem they had was all 4 woofers needed surrounds also. They were marked for $20... EACH. I walked out of there paying $20... for BOTH!
I am now in the process of refoaming all 8 woofers & came across this thread again. Apparently, there is a debate on which series 601 sounds best... Series I, II, or III?? The vastly different physical designs have been noted, but not explained as to actually why Bose changed from series to series. I have some insight here which may help some to understand better why the 601 Series III sounds better & sells for more money on the open market.
The 601 Series I is the shortest version (with probably less internal air space for the woofers than Series II or III, I did not do any calculations/measurements) that uses a vertical round port/tube (which is located right in the middle of the tweeters/mids & upper woofer & causes interference with the mids/highs & "chuffing" when played loud with a good amount of deep bass) & a more "congested"/bulky upper grille area (which also interferes with the sound). (I believe it has a REAL woodgrain veneer, which is good.) Plus, it fires the tweeters/mids (called "twiddlers" by Bose) at a very low level, not ear level, which will most likely negatively affect the sound (stands would be needed to correct this).
(Result= a good sounding speaker, just not refined as well as it could have been before being released to the public.)
The 601 Series II was the improved version due to it's taller cabinet (which probably has more internal air space for the woofers, probably allowing them to play deeper bass & places the "twiddlers" at a higher level... closer to ear level for better imaging. (Unfortunately, it had vinyl veneer, not REAL wood veneer like the Series I) The upper grille area was also relieved of some of the bulk & physically opened up a little more in order to get cleaner mids/highs than the previous model. However, the Series II again used the vertical round port/tube placed right in the midst of the "twiddlers" & upper woofer, which causes interference with the mids/highs & "chuffing" when driven hard with deep bass. This vertical round port/tube also blows upward against the cheap plastic-framed grille & grille cloth, which can cause sonic vibrations/rattling of said plastic grille (which, by the way, looks much nicer than the previous series). It's base is also larger than the Series III & takes up more floor space... don't know about the Series I?
(Result= an even better sounding speaker than the previous model due to larger cabinet volume for deeper bass, higher level of "twiddler" placement which improves imaging due to being closer to ear level, & nicer looking grille shape/material. However, Bose unfortunately took a step backwards by NOT using REAL wood veneer like they used on the Series I, and that was such a shame.)
The 601 Series III was the final improved version of this design. It had an even smaller footprint than either of the two previous series (greatly improved the WAF - Wife Acceptancy Factor); it was taller than either series which put the "twiddlers" at proper ear level for the best imaging; it still had a large woofer cabinet volume ( for deeper bass); it no longer had a huge grille on top but instead had a much more sonically transparent, easily removeable, non-vibrating grille; it NO LONGER had a vertical round port/tube (which is probably THE most important & effective improvement yet) but instead a SLOT PORT located on the rear backside bottom of the speaker (which eliminates ANY interference with the mids/highs sound quality & ELIMINATES the chuffing noise PLUS allows speaker placement to use the back wall as a sounding board which can augment the bass even more, if desired/needed in different home's applications. Just research slot ports versus round ports and chuffing. The high-end car audio company, JL Audio, ONLY uses slot ports on ALL of their custom subwoofer boxes... I guess they don't like the chuffing noise either? Even Altec Lansing used slot ports on their 604 cabinets! There is DEFINITELY an advantage of using slot ports over round ports; on the Series III's; Bose even uses slot ports (not round ports/tubes) on other Bose speakers, like the 201, 301, etc...; Bose also went back to using REAL wood veneer on the 601 Series III cabinets... YEAH!!; the Series III also have two different colored grilles... interesting design, but works for most.
(Result= the best sounding series of Bose 601, or for that matter, any other Bose speaker ever made,... period. It had more contemporary design than older series, better sound quality, higher WAF than older series, more power handling than older series (I think? 200 watts versus 150 watts), better imaging, better upper grille design, REAL wood veneer,... Yes, some people think the Series I or Series II are the best, simply because they own them (biased) or because they heard the Series III with cheap equipment, in a room not suited for them (non-reflective walls) or simply placed improperly. But with all factors equal, set up all three series in the same, proper room for them, with higher-end powerful electronics, and the differences between them are obvious. After all, why would Bose, a major manufacturer, change it's manufacturing/tooling process (which is very expensive) if the changes to the speaker were not better than the series before? ** Wait! I know what some of you are thinking... "Well how do you explain Bose changing from the 601 Series III to the 601 Series IV, because it sounds like crap compared to the Series III? That was a move backwards!". Yes, this is correct, but many companies make wrong business decisions to satisfy a certain niche market (in this case, "housewives who don't like/want big floorstanders or even slightly larger bookshelf speakers in their house & they wear the pants in the family").
(Yes, crap happens but sending American jobs overseas & having cheap products sent back is not our own personal faults... darn politics & greed! That's a topic we need not start here. It simple... if we bring back the jobs to the U.S., the quality of products "Made in America" will undoubtedly improve.)
I hope this helps shed a little more light on the subject's of which series sounds better or why the Series III gets all of the glory/attention & commands the highest resale amount of the first three series & basically ANY Bose home speaker. You have to try and picture in your head the Bose speaker design team back them comparing designs of the Series I with the "new & improved" Series II. Then, later, the "updated" Series II with the "new & even more improved" Series III. When the Series III was being developed, they probably even tested all three together! ☺
P.S. To answer another question about 501 replacement woofers. I owned a pair of 501 Series IV which needed new surrounds when I got them (free from some guy who just wanted them out of his attic). I was younger and didn't know how to re-surround speakers yet, so I called Bose & ordered 2 new woofers ($35/each?) & new grilles ($25/each?). When I went to install the new woofers, I installed one in the first cabinet & decided to do an experiment with the other cabinet. I installed a Soundstream SPL10 (a car audio sub) into the other cabinet, just to see how it would sound. The SPL10 (a 10" woofer with a huge magnet compared to the anemic Bose woofer) had dual 4-ohm voice coils while the Bose woofer was a single 8-ohm voice coil. So I wired the SPL10 in series for an equal 8-ohm load, to keep everything as close to the same as possible. When I A/B'd the two cabinets, I was not expecting the SPL10 to have the same sound quality the Bose woofer had... more bass, most likely, but less quality. Man was I wrong! It made a HUGE difference... Cleaner, tighter, deeper bass! I was simply amazed at the difference. I still installed the factory Bose replacements, but have never forgotten how it sounded with the SPL10. A lot has changed in the design of woofers in the past 30-40 years & upgrading to newer, technologically better woofers can greatly improve sound quality & one's listening experiences. Yes, I can hear it now... "If you change woofers, then it's NO LONGER a Bose 501 Series IV!"... and those people are absolutely correct. However, it IS a better sounding version of it. And yes, others are thinking "You have to take in the Theile/Small parameters... measurements... do crossover changes... impedence matching... sensitivity matching... correct for less internal air space due to huge magnet size...blah...blah...blah. I know all of this, as you do, but sometimes ya get lucky and have something on hand that works perfectly to YOUR ears, & isn't that what this "hobby" is all about? OUR own ears, not other people's ears? Yes, in a perfect world we ALL would have been trained in electronics, have the proper testing equipment, have access to whatever drivers we needed, but this is NOT a perfect world and we must try... experiment with different things sometimes... and most importantly, HAVE FUN DOING IT!! I had a blast swapping woofers that day! Hearing the difference in woofers... learning for myself... repairing something that was broken... it's a hobby & hobbies are something to do (which you like or are interested in) for FUN when you are not working or being nagged by someone... it's a getaway from the stresses of daily life. There are no ABSOLUTELY SPECIFIC rules to ANY hobby... if there were, it would feel more like job instead of a hobby. So go do some experimenting... play around with some old equipment... learn how to solder on a cheap, old vintage receiver... get glue all over yourself while resurrounding some woofers... bring back old stereos/speakers from the dead & breathe new life into them... fix some vintage equipment & then give it away to someone who would/might appreciate it (a young kid... older man/woman... someone who only listens to crappy rap music [LMAO!!], whoever you know that has a cheap, "Made in China" stereo...)... live life without always having to be a perfectionist... there is no perfect speaker, no perfect sound... It's all about the music!! After all , the MUSIC is why we love this hobby in the first place!! May all of your music sound good through your equipment to your ears... except rap... everything but rap.
But from reading online I get the impression, that that is not how to rule out that the units will work together. I'm in no need of blasting off the roof, but I like to fill the living room with a folk-record at a volume level, that will win over the db of two kids playing and a dishwasher running.
