Stereo Subs - Do they need to be identical?

I would say that... it depends.

In my opinion the goal is to get a smooth in-room response in the bass region, and a smooth (perceptually seamless) blend with the main speakers. Depending mainly on the frequency response characteristics of the subs and their low-pass filter, their positioning, and the rolloff characteristics of the main speakers (whether or not they are high-pass filtered), it's possible that in some situations identical subs would give a better blend, and in some dissimilar subs would give a better blend. In general, I would expect dissimilar subs to be a bit smoother. Let me explain why:

In my opinion, most subs are not designed with room gain correctly taken into account. Here's a link to a graph that I think was generated by Martin Colloms, depicting "typical" room gain (roughly 3 dB per octave below 100 Hz):

http://www.speakerbuilding.com/content/1020/rge.gif

So if you take a sub that's "flat" to 25 Hz (anechoic), in-room it may well be +6 dB at 25 Hz - which will sound heavy and boomy.

Now suppose you have two subs, one of which is flat to 40 Hz, and the other flat to 25 Hz. The summed response of these two subs may well come fairly close to approximating the inverse of room gain.

If you start out with subs whose anechoic frequency response is approximately the inverse of room gain, then I think two such subs would be smoother in-room than two dissimilar ones.

Another factor to consider is where the subs will be placed, and the slope of their low-pass filter. Placing the subs far apart (but not in corners) is usually beneficial if smooth bass is the goal, but the farther the subs are from the main speakers the more important it is that they have a steep-slope lowpass filter so that lower midrange energy doesn't come through loud enough to give away their location. I personally favor highly asymmetrical placement, but if the crossover is high and/or the slope shallow that might not work well.

Duke

Using two different size subs can be very helpful in evening out room nodes. Place them at opposite ends of the room. With monitors as mains, I would use two identical subs. Proper integration is way more than twice as difficult, but the effort is worth it.

As you go towards small ported subs you tend to get more harmonic distortion thermal compression, lower overall SPL output and poor transient response. It is just physics and heavy helpings of compromise. Larger is better when it comes to subs. Sealed box usually has the best transient response even if output SPL is correspondingly lower.

Two subs or one - I don't think it matters that much - for best sound I would not recommend to place a stereo sub next to each main speaker. Definitely an asymmetrical placement will be better.

Again, if using monitors, stereo subs next to them is definitely the general recommendation. And it most assuredly does matter that much!

Again, if using monitors, stereo subs next to them is definitely the general recommendation. And it most assuredly does matter that much

The reason I suggest to avoid stereo sub placement like you describe is to
avoid quarter wave rear wall cancellation in the bass that you always get from symmetrical placement of full range speakers. The great advantage of a sub is that you can reduce this problem with an asymmetrical placement.

I agree that stereo subs next to speakers is definitely the "general recommendation" though. With so many free standing full range speakers out there, I suspect most people don't worry about these details or are blissfully unaware. No wonder there are so many room acoustic bass problems and speaker demos never sound the same twice and some companies have special "setup" technicians trying to figure the best sounding compromise in placement of large full range speakers.