A HSU VTF 15H, with 2 or 4 Soffit Traps from GIK Acoustics and a miniDSP will beat any subwoofer without them for musicality, depth and dynamic range.
The biggest problem in getting subwoofers for high quality loudspeakers is not really the quality of the sub but the integration. The room acoustics behave differently below 200 Hz and especially blow 40 Hz, so adding bass extension moves audiophiles into new territory. Your room rings like a bell at low frequencies, and the bass traps are necessary before EQ will work.
Given a choice of a $40,000 subwoofer "naked" or a single $1,000 Hsu with bass traps and EQ, I'll always take the latter. In fact, I wouldn't do a 2-way speaker without bass traps after hearing them. By the way, Hsu makes very very nice subs for the money. I'm not trying to knock them at all.
Best,
Erik