Stereo Subs - Do they need to be identical?

I will have to weigh in with an opposing view. I have had good luck using 2 totally different subs in the same system. YMMV, but I can see no reason not to try it, especially if you already own one sub and are adding a 2nd.

I don't think there would be a problem using different subs as long as you are crossing them over low enough but I would go with the same size myself. I don't see any merit to the smaller sub being faster theory. First of all a small sub has to move further to produce the same amount of bass as a larger sub therefore negating the speed advantage. In addition bass is slower because it is a longer frequency. If it were fast it wouldn't be bass. A bass drum is larger than a snare for a reason.

I have found no correlation between size and performance in subs. My subs have 10" and 12" drivers and go very low. The idea of a big slow cone for a woofer was prevalent when I got into audio in the early 60s. Then you had the Electro-Voice with a 30" woofer and the Hartley with a 24" woofer. 15s were common in 3 way systems. As speaker design has progressed woofers have , by and large, grown progressively smaller. Now the $33,000 Avalon Diamond , which may be the best speaker in the world, uses 2 7.5" woofers. It is the overall design of the sub that counts , not the diameter of the woofer. I would not worry about mixing different sizes, AS LONG AS, you are crossing them over sufficiently low. I use 22 Hz on mine. If you could hear a difference between them then you would be crossing too high, you should NEVER be able to hear a sub, just feel them.

There is a reason subs can be smaller now but you still can't defy the laws of physics. A small sub in a small box requires much more cone excursion than a larger design. For a small sub to equal the output of a large one it takes much more power and also equalizing to go as low. The driver also has to capable of much greater movement. Computer aided design and servo control has been a blessing when it comes to sub design. Also digital amps allow high power with low heat and low price. Crossing over as low as posible is the secret to seemingly fast bass response not the size of the sub.

If you can "hear" the sub, as opposed to "feeling" it, then different sizes should be discernible to your ears as differences in tone or volume. Therefore, I would prefer to go with identical subs. If you do use non-identical subs, I think that the crossover frequency should be quite low so as to avoid hearing any differences. Keep in mind that the open E string on an electric guitar is somewhere in the mid 30's in terms of Hertz. I wouldn't want to crossover any higher than that.

As far as sub speed goes, my experience is that the higher the crossover frequency, the slower and more sluggish it makes the main speakers sound. How fast a sub is depends on a lot of design aspects in addition to cone size so I wouldn't feel comfortable making any generalizations. Speaker designers can tell you a lot more than I can on this point so I'll defer to them.