Stereo Subs - Do they need to be identical?

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.

Following the advice of mfgrs who know much more than I has worked too many times to count. Perhaps you are using monitors too close to the back wall. In theater applications asymmetrical is the way. Please try what these folks (mfgrs) recommend first before dismissing it based simply on theory.

Thanks guys for all the responses - a lot to think about. I have a key, related question. My MBL monitors have a low pass filter that cuts the bass off at 49hz - that's the lower limit of the bass.

Were I to introduce two subs to the mix, where would you recommend I mark the upper crossover point for the subs? And, would it be worth introducing some external crossover to cross the MBLs even higher (not force them to go as low as 49hz). Finally, even then, would some room correction unit be a welcome addition (like the Copland DRC) - maybe I'm straying too off topic here, so would need a seperate thred, but I would like to resolve these questions together and they seem tightly related. Overall, I'm looking for absolute seamless integration for two channel music. I had thought of getting 12inch subs because I may move to an apartment in the near future and I want to make sure I don't overpower the room.

To answer one of the questions above - my MBLs are about 4 feet from the back wall and three feet from the sides - its a large open plan room. I hope to upload updated system photos later this weekend (after I tidy the room ;-) )

Thanks,
John.