"Each sub may have the response you mentioned, but what happens when you combine them? I believe they interact and have different responses when used together.
From what I've read, I don't believe they just sum, although now that I say that, it doesn't sound right. I believe subs are linear systems, so superposition should apply."
You are absolutely correct. I went off half cocked. One can't separate phase and magnitude (out-of-phase signals produce a null). Etc. It's a lot more complex than that. Just how complex is apparent here:http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf
I suspect that Sound EQ can do a decent EQ with two subs on one EQ channel or with each sub on its own channel. In either case the EQ feeds a known signal, measures the results, and designs a filter to correct any problems. I lean toward the second option though, partly based on this example:http://www.audioholics.com/productre...eyMultEQp1.php
and partly because with two different sub locations, I think one channel per sub will break the task down and effectively apply more processing power. I.e., if each sub can be individually corrected to provide flat response and the proper phase, then the two subs should look the same from the seating area and thus provide only an SPL increase when the second one is cut in.
The importance of two subs being equidistant from the listening area is stressed here (last few paragraphs on page 3):http://www.audioholics.com/techtips/...ProPrimer3.php
Clearly Sound EQ can't correct the huge time error from an out-of-phase sub at 20 Hz so it's important that both subs be not only equidistant but about the same distance as the mains from the listeners. Given a reasonable starting condition, I suspect Sound EQ can achieve near perfection.
Interestingly, the Harmon article on page 22 presents a situation where two subs is problematic and removing one (or stacking it on top of the other one) is better. Of course, the author didn't have the benefit of Sound EQ.