In my HT, I've found the limitation to be the room itself. Above a certain volume, things start to vibrate sympathetically in the room. This "distortion" is more annoying than a lower amplitude for the sub. I DO "batten down the hatches" as I figure out what's vibrating but, so far I haven't gotten everything quiet. It reminds me of the car/boomboxes with the trunk lid vibrating quite loudly. (How can those kids listen to that stuff?)

Old Man JMS