I have two subs for a few reasons:

1. Large room and 7 surrounds to keep up with.
2. Less room interaction. Well, probably.
3. It looks better (symetrical). To me anyway.

SQ? One could argue that two drivers have to work half as hard and therefore distortion is lower. I don't know how much difference that really makes from 20 hz to 60 hz.

I know this is not the technical answer you were looking for but maybe someone else will chime in.

I'll be on the pre-order list for the next 990!

Rick