Hsu Research is not as well known as others, but those who have owned/heard them are highly complementary towards them. His are "true" subs in that they dip down to really low frequencies, unlike some of the mass market so-called subwoofers that are IMHO nothing more than thump boxes. And I think Dr. Hsu's low marketing costs make the subs a true value--they are reasonably priced yet can stand up to subs that cost substantially more.
Martin Logan also has a respectable lineup of subs. There are some nice models from their past that represent a good bargain if you can find them. The reason I mention them is that they voice them to blend seamlessly with their other products (and primarily with panels, they don't want to cloud the clear midrange and upper bass with "murk"). I'm not certain if their Dynamo series is up to the task, but others I've seen (Depth, Descent, Grotto, etc.) do go quite low and should handle the bass cleanly. Some of their current subs offer wireless connection, so there are fewer wires dangling around the room.
I also agree on using two subs vs. one, but placed asymmetrically in the room so that standing waves (and null points) are reduced. I am headed that direction eventually, but with perhaps three or more smaller subs (think of it as a poor man's distributed bass array system--strategically placed, they can really even out the bass response throughout the room).