Originally Posted by nube
(Apologies if this is preaching to the choir or pedantic - I haven't been keeping up with the thread.)
It's been my experience that, when dealing with the plethora of really good (SVS pc) to world class (PSA triax) subs, carp is 100% right. Your old sub likely didn't create a lot of distortion during normal playback unless you were severely overdriving it and, even then, SVS makes some pretty bulletproof stuff, so that wouldn't be my first inclination.
Rather, I'd think you're hearing peaky frequency responses. Loud peaks make a much larger impression on us than the levels of distortion you could likely see from that old SVS sub. We train ourselves to like peaky frequency response over time, as our ear-->brain connection is wired such that louder almost always sounds "better," "fuller," or "more," especially when it comes to bass frequencies.
I suspect that a lot of people experience this, simply because they have taught themselves to think of it in this way by upgrading, not EQ'ing flat, and automatically equating the new higher dollar sound with better sound - which may not always be the case. We always want our upgrades to be upgrades, right? Once you get used to a particular response, that becomes your expectation.
If you EQ flat and level/gain matched to your other speakers, it will sound weird (and probably not good) for a while. It takes a fair amount of time to retrain the brain to hear the nuances of a flat frequency response. However, I guarantee it's worth it!
Also, on a related side note, I wouldn't trust any receiver's room correction software to actually get it right without unsmoothed REW graphs that verify it as such.