The odd numbers are the major upgrades. The I and the II are almost alike, the III and the IV should sound exactly alike, the V and the VI are almost alike. Or you might consider the Series III to be the only radical change.
The I and II have fabric surrounds, treated with oil, which seem to last forever. They have 4" drivers. I recently found out that late Series II's have foam surrounds. The cabinet is also made out of solid wood. I've never heard the early ones. Some people say they were the best. Other people say that they lacked detail in the bass and smoothness in the treble, compared to newer generations. The I and II went deeper in the base, if one doesn't engage the Bass switch on the EQ. By cutting off the bass below 40hz, that control reduces the required amplifier power.
The originals and the Series II were flat in front. They had cloth grills, but with an optional walnut cover with a gap for the front driver.
The Series II was available in an edition known as the Continental, which should sound the same as other Series II's, but has a curved front, so it resembles the later Series III and IV, but with a corded grill.
The Series III and later have 4.5" inch drivers. The early ones are known for being power-hungry. this was fixed with the Series III, by giving them a more flexible voice coil, made of flat (helical) aluminum. The III also introduced the plastic internal compartment known as the Acoustic Matrix, in part to improve the bass. The ported designs have attenuated bass below 38hz (without the need for a switch). They also have wind noise, which isn't noticeable as noise, but which can make the bass sound somewhat muddy.
The original acoustic matrix was enclosed only by the particle board cabinet. The later ones are enclosed in plastic, so theoretically, you could remove the outer cabinet and still play them. They made this change in the middle of a Series run. I don't recall if it was when they were making Series III or IV.
The V added a coating to the back of the cones, to smooth out the low treble and high midrange. This ostensibly gives you more natural female voices.
To my ears, the VI do sound subtly clearer than the IV.
The V had aluminum on the sides; nobody liked the looks, and some also claimed that the aluminum sides hurt the sound. That was the main reason they were replaced by the VI. The VI also had changes to the equalization, partly to adjust the sound to compensate for the change to particle board and wood veneer on the sides of the cabinets, partly because Bose "performed research" (changed their minds) about the acoustics in an average home.
If you have the III, IV, or early V, or II with foam surrounds, then by now the foam surrounds should be brittle, unless, of course, they've already been replaced. If you don't do the work yourself, replacing the surrounds is expensive because each speaker has nine of them. Later V's and original VI's have improved foam, replaced with some antibacterial substance to imcrease their life. Now Bose is selling the Series VI Version 2, which returns to cloth surrounds.
Bose has a transferable trade-up policy. If the foam surrounds go bad, you pay for the shipment to Framingham, Massachusetts. For a discounted price, which keeps going up, you get new speakers and equalizer. they tell you to keep your present pedestals.
(I wonder what they do with the old speakers and equalizers.)
I used to own the 901 Series IV. I asked a Bose representative if I could upgrade them by replacing just the equalizer. His answer was no, that would make them spikier. Each generation of EQ is optimized to each generation of speaker. However, as I pointed out before, there is relatively little difference between EQs of Series I and II, between III and IV, and between V and VI.
Suffice it to say that the Series I and the Series II are *basically* the same speaker, and the Series III, IV, V and VI are basically alike. The III and IV had with an extra connection for the Bose Spatial Control Receiver, which permitted the user to attenuate either the inner or outer rear banks, to widen or narrow the image.
