I've heard very good things about Rythmik Audio subs, but have never heard them personally. These are kits and you gotta build em, though. If you search, I think other internet vendors offer them as complete units.

In any case, all the above suggestions are good! Like Rick (wid) recommended, the Hsu subs are fantastic. I own a VTF3 MkII and it's been a spectacular performer.

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose