Unfortunately, htshack.com seems to have removed most of its subwoofer test database. By memory, I wanna say that the 8" subwoofers typically produced about 4 or 5% THD at 50hz at 85 DB (in a quasi-aechoic set up that effectively removes room nodes). At 95 DB, that number more than tripled. At lower frequencies, performance is much worse.
Subs with larger drivers fared much better. I switched from a pair of 8" Velodyne SPL(?) subs to a pair of Rythmik 12" subs and the A-B comparison was pretty compelling to me. The ear is pretty insensitive to THD at these frequencies, but the sound "cleans up" in a very nice way with bigger drivers IME.
I'm pretty sure that there's a database out there somewhere with more detailed info, but I couldn't find it on a first search and I'm too lazy to do another.
