I think it would probably be ok using the controls on a sub (maybe one of the smaller Rythmiks) to integrate with the HTM200's. I'd try that first before investing in another receiver/preamp.

Depending on budget, if you decide you need another receiver/preamp, might want to look at a miniDSP (e.g. the Flex) to use as a x-over and bass management/eq. I did this with my dual E15s and ELX RAAL Towers for my 2 channel system although the system was pretty good without the Flex (but a little bass heavy at times, which you'll likely not have with the HTMs). The other reason I added the Flex was that I could set up 4 different profiles for music -- 2.1, 2.1 with house curve, 2.0 with bass EQ (so I could use the Towers by themselves), and 2.0 w/o any EQ.