I had to chime in, as this hits me in the personal life history department like so many I've read posting here. I have a few tech notes about the bass of the Mach's to follow.
In the 70's, when I was a teen in high school, every Radio Shack had these playing something. At that time I befriended the local RS manager (had dinner at his house, helped him move, knew his girlfriend, my girlfriend got a job in his store, etc).
The Mach 1 (2 and 3) were a marketing triumph for RS. They sold millions of them.
Many of us have fond memories of the era, and there's a particular sound color from Mach 1's related to their marketing influenced design that evokes that memory like the scent from mom's kitchen at thanksgiving.
When you like them, so be it. It isn't because they're particularly great, but because they were a phenomenon of that era, leaving an imprint because they became a "thing" in our life history, like the TRS-80, the Apple II, the Triumph spitfire or other iconic products of the time.
I never owned a pair. I heard them in the store as I visited my girlfriend, browsed for electronic parts, or helped fix up the display for in-dash replacement radios and CB's (not a typo, CB radio, the era's version of cellphone for short distances while driving), for hours at a time, for at least two years, not quite every day.
The bass is a bit weak, technically. The bass driver is a high Q design, meaning (colloquially) it has a large bump at resonance. The box is a tad too small, but tuned right below that point where you'd pump the bass control anyway, so it "fit" well for a good, solid "boom" at medium acoustic power. The xmax is below 3mm, so the driver doesn't really move enough for good SLP below 50hz (should have been 4mm or more for the era, 8mm minimum for the 21st century). That fit well with LP playback (to avoid rumble and low frequency feedback - a "feature" of most speakers of the era).
This was a marketing influenced design. Customers looking for a solid, big sound at a particular (full retail) price point, who otherwise didn't have much knowledge about audio systems would be easily impressed by the particular selection of music in store, the placement in store and that swirl on the driver. They sold millions at a very high profit margin.
They are not highly accurate, flat, or powerful. They did sound good to most people.
A number of brands copied the idea, selling an even cheaper implementation of the basic notion of a big cabinet (relative to a family setting of typical home decor), with a big sound (tuned to boom, just enough to be 'nice') and clarity (the horn loading was effective at 'presence' - many copiers didn't bother). I picked up a pair of Jensen CS 315's (for free) which seem to be a market designed product intended to catch the kind of buyer that would purchase Mach 1's but didn't have enough cash, and are much more cheaply made overall. The Jensens have a 15 in woofer in a box barely large enough for it, perhaps a 4mm xmax and a sloppy Vas, which give the impression of a big sound (a marketing notion) and a big box, but probably sold 25 to 50 percent lower retail point with a very high profit margin (I mean, the crossover in the Jensen is just a capacitor on the midrange, and one on the tweeter).
In that way, the Mach 1 was quite superior.
Now, the more discerning you are, the more critical you'll be. I was a classically trained violinist in my youth (ended up as a computer programmer, so I dropped the violin study in my 20's), but I listen to a wide range of music from Haydn to Mahler, Beethoven and Debussy, Van Halen to some modern stuff I can't even name. I never, in my whole life, listen to music at volumes which would injure my hearing, and never attended concerts of rock or pop because of that self protective interest. As such I retain a very critical view of any system, but I have plenty of tolerance for idiosyncrasies because all speaker designs are compromises.
Mach 1's have plenty of idiosyncrasies, and the motivation of the design is about the psychology of sales to the masses more than anything else. It is, however, one of those creations which, perhaps accidentally, landed in a sweet spot that provided a very satisfying result regardless of those design compromises one might consider flaws, some might prefer to call character, or color.
Over the decades, and starting from projects with my father as a child, I've built my own as a hobby, informed by the science (and appropriate testing gear). I built many pairs, with 15 or 12 inch woofers, carefully selected (balanced regarding cost/performance) mid's and high frequency drivers which I would regard is vastly superior to a pair of Mach 1's (with testing/measurements to back up that claim) than a pair of Mach 1's.
Nothing, however, will evoke the memory of that first long term girlfriend, the scent of the Radio Shack Store, or of the first Star Wars, or the release of Star Trek "The Motion Picture", or those particular haunts I frequented, that Triumph Spitfire I drove, or the halls of my old high school quite like a pair of Mach 1's, and that swirled cone.
Even if I don't favor them particularly, I'd pay $100 for a pair just on sentimental value.
