I'm really concerned about those you who are mixing dissimilar dual subs in hope of getting better even frequency responses in your theatres. Ed Mullen already said that each sub has different phase response and mixing the two dissimilar subs will cancel each others frequency response creating a combing effect in the frequency response. It could be skewed one way or several limited due to the phase cancellation going on. Say the 40Hz frequency from Sub1 will have positive polarity( or a lorger phase angle due to the electronics in the sub) but that same 40Hz from Sub2 will have a larger phase angle or have negative polarity depending on where you've placed in in the room and the combined cancel out creating a null that you can't get rid of at all. Unless you have a unit that phase/time aligns your subs like the AS-EQ1 which SVS sold(but I have) but sadly discontinued, you are simply out of luck.
I'm not familiar with Audyssey tools, so I not sure if they time-align multiple sub, Others most familiar with Audyssey could chime in here ....
Would you mix your fronts mains that way? would you have a Martin-Logan(ML) Electrostatic(Summit X, $14K) for the left front and an ML Electro-Motion for the right($2,195)? Both individually sound great for the rooms and audiences they were designed for but together in the same room, the sound wouldn't be so great. The tonality would be way off. NO amount of level EQ or PEQ would get them to sound great together.
Another case I point ....We all also know that we don't use speakers for the center channel and the mains... The tonality (mainly from the center speaker) would be so very different from the mains that no amount of EQ would solve that problem at all. And since most sound for movies is mixed into the center speaker, it will stand out like a sore thumb and you'll hate it.
So, I implore you to use similar subs if you are using dual subs designed with the same phase and amplitude response. I know costs are a huge matter but since you've decided to go dual anyways, you've already made the choice to spend the extra $$$ for the sub.
I personally have dual SB13U and my 1300sqf room and they are indeed superb. Even at very low listening levels, when I'm working on my laptop, I hear gentle, deep rumbling that seemingly come from nowhere ... and this is from HD cable TV material!!! On HQ Bluray material, its just awesone.
SB2000 were extremely well reviewed a few months ago as well so these would make an super and excellent pair as well. I'm sure their ported cousins(PB2000) would perform just as good if not better.
Sorry for the long post but I feel so strongly about this matter. Yes, you may want to save money overall, but you end up compromising your home theatre system in the end.