Stereo Subs - Do they need to be identical?

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.

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.

There is much to be said about a simple system but 49 hz just isn't great for rock or classical. A sub crossed over at around 50hz with a steep slope should be very non directional and integrate well in your system.