I think the main issue with differing subs if they perform differently at the higher (or lower) frequency ranges. For example if you have smaller speakers with a X over at 120Hz, and 2 subs but one of them can't play well up to that 100-120Hz region, you'll have a relative dropout in that region. Differences in output should be able to be compensated by the gain level on the sub itself. I have no proof of this, it's just my logic at work here... I assume someone actually knowledgeable on the subject will chime in soon.
