I haven't purchased any speakers yet, aside from my subwoofer, which is an Outlaw LVM-1 EX 12 inch downfiring sub. It's got some volume, and shakes my much larger family room upstairs with clean bass. I expect it to fill the space downstairs nicely.
[edit] And my after 8 hours at work brain just realized that you're referring to transducers on the seats, I believe. I'd like to hook those up too.