2 Car Subs - 10"
If you're going this route, keep in mind that speakers come in different impedance ratings, and amplifiers have minimum impedances they can support. Typical car subs have 4 Ohm impedance rating, while typical house stereos require 8 Ohms minimum impedance per channel. There are, of course, 8 Ohm car subs, and house stereos that support 4 Ohm loads, but you have to actually look for them.
Some other things to consider with car subs:
- car enclosures are generally carpeted in a neutral grey to blend in with the trunk carpeting; this doesn't look nearly as nice as a well-finished wooden house sub box
- car enclosures are typically trapezoidal rather than square, as they are normally built to wedge against your back seat
- generic car subs and generic car enclosures are not specifically engineered for each other; house subs and their enclosures are typically specifically engineered for each other
Given your current set-up and living room size, I'd say look for a 10" powered sub with 100 to 150 watts. Look for level and cross-over adjustment knobs on the back so you can finetune how boomy your sub is.
I like downfiring subs because I have kids and it's almost impossible for them to break the cone or put things in the port, and also they look better, to me (I think music that just magically appears in the room is better than looking at a bunch of speaker arrays).
As for brand names? I can't even remember what I bought three years ago. It's given me no troubles, so there's been no need to look it up. Don't bother with Sony subs. Pioneer might be alright, although probably pricey. Klipsch is a name I've heard before, and Energy (although I've never heard their subs).