Thanks for all the replies, so far!
To answer to cel4145's questions...
Budget is somewhat flexible, as cheap as possible would be nice, but let's say an MSRP of under or around a thousand US. I'm pretty good at scrounging deals (I picked up the Kef's BNIB for $795 total), so I intend to realistically pay somewhere in the neighborhood of $150-$400, depending on whether it's a deal on a new sub, a used one, what the original MSRP is, etc. etc.
As for size, I'd like the overall footprint to be as small as possible, just for aesthetics. That said, I can definitely get away with something larger than the microPRO which is roughly a 12" cube... That 14" NHT B12D sounds great, assuming it has the power I need. If I were to go for something truly large in size, maybe a wireless option?.. When I was building the Polk setup three or so years ago, it seemed wireless subs didn't get as much respect on the forums as their wired counterparts... Lately I seem to see a lot of good reviews, so maybe the tech has gotten better?
I listen to probably 60/40 HT to music.
Dual subs... Right now, the sub is next to the credenza, and being that it's a good-looking piece of equipment, it doesn't offend. I don't have enough room on the other side of the credenza for a matching sub, due to the stairs to the mezzanine. That said, if we're talking about a small wireless option, then two subs would definitely be an option... I could place them on either side of the couch.