I need a subwoofer recomendation

I've got a couple KEF Q900's powered by a Yaquin tube amp and primarily play records. I'm looking for a sub that might also work for movies down the road if I ever put together a surround system. The sub needs to have speaker level inputs and I'm looking to spend $500-$700. My room is 20x20 with a vaulted ceiling and hard surfaces. I've been looking at the HSU VTF-2 Mk5 and the SVS SB-1000 and SB-1000 pro. Is there anything else I should be considering?

On a separate note, if I head down the surround system path, I was thinking of starting with the KEF Q250 or Q650 center speaker to start. Is there something other than KEF I should be looking at that would be a decent match with my Q900's?