high end subwoofer...which one tho?

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...forget about the Velodyne DD-18 as someone suggested here. It is waaay to much overkill for a room your size.
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If price isn't an object (nor is floor weight), check out the new Paradigm Sub Two. It has six speakers and something like 3400 watts from its amp. The initial reviews are amazing. They have apparently aimed right at the high end REL and Fathom buyer.

I use Rythmik subs, but for a dedicated HT set-up, I'd probably choose SVS. Disclaimer - I use a pair of 12" Rythmiks (not 15" which are probably more suitable for HT use). Also note that my comments are based on comparison of the performance test data I used to choose the Rythmiks, NOT on any actual listening session comparing the two brands.

The test data were useful because they illustrate the difference in the design priorities between Rythmik and SVS. In a nutshell, SVS optimizes sub bass and Rythmik deep bass. For reference purposes, in a 12" model, the SVS start to pull away below app. 35hz (anechoic).

The Rythmiks allow maximum control over the driver's damping and this allows a lot of flexibility in optimizing the match to mains for music. The design trade off (vs. a similarly priced SVS unit) is that there's a bit less clean SPL capability, which may well be audible on an the sub bass info in an effects track (though it's very unlikely to ever be a problem for music). I would (and did) prioritize the former for my 2 channel music only system, but I'd go with the latter for HT use.

Again, YMMV.

Good Luck

Marty

One quick additional thought:

In a smallish room, you're not going to need a monster sub - and a 15" model from any of the brands that have been recommended here will likely provide crushing bass capability in your room. The 12" SVS cylinder is probably more than enough.

As Bob pointed out, the addition of DRC (like the Audyssey chip included in many HT receivers and pre-pros) will almost certainly be extremely beneficial to bass performance in your space. The difference between DRC/no DRC will likely dwarf the difference between choosing the best sub versus choosing a merely good sub.

Marty