Over on a forum in the Uk is a Fin (moderator edit: Ilkka) who has both an understanding of how a sub works and how to measure one. He has posted a couple of sub test threads from tests he ran during the 4 or so hours of daylight that Finland gets each year. He doesn't make too many subjective comments as they are, well - subjective. But it does provide a good comparison of a range of subs available in the USA and Europe.
You'll get a look at the BK Electronics subs, whose value for money is undoubted. They OEM manufacture subs for the likes of REL, MJ Acoustics and Ruark. For comparrison, the XLS-200 is a direct competitor for the REL Q-200 but retails for £289 against the £599 of the REL. The Monolith which you'll hear mentioned from time to time is also here. Over here it is very slightly more expensive than the SVS PB10-ISD but boasts more flexibility and a wider range of room friendly finishes. They are directly comparable. BK sell online only.
The BK XLS-200DF, BK Monolith FF, SVS PB-12 Ultra (20, 16 & 12Hz tunes) and SVS 20-39PC+ (20, 16 & 12Hz tunes) are
here.
Part 2 contains the SVS PB10-ISD (ported and sealed) and the Axiom EP-600
here.
Part 3 is due soon, but they're still waiting for sunrise.
Russell