Thats a pretty d@mn good size room. 7650'3
I should ask, probably, are you here in the States? (availability)
Next... budget?
So, have you checked out information on subwoofer set up? Especially the
Bassaholics guide here? All of the subs reviewed here usually get a rating for the size room they can handle. A helpful starting place. In terms of actually setting up multiple subs, there is a lot more than just plug and play involved to make them play nice in your room... BUT ITS WORTH IT!
OK: Most of the subs that get recommended are very good subs for a pretty reasonable price. For your room size, I definitely would recommend ported subs, as they will tend to have higher output, and can dig a little deeper in frequency. You will need to consider some good size subs,13-15". Likewise, I think all of these are good as multi-use subs... music and HT.
Depending on the sub, and the connections it has, you will either be daisy chaining one sub to the first one plugged into each of your sub pre-outs, or you will have to split the pre-out with a Y and then run the cabling for all subs... but still two per sub channel.
Hsu Research VTF15H Mk2
Outlaw Ultra X13
Monoprice Monolith 15"
SVS PB4000
SVS PB16 Ultra
Rythmik FVX15
Rythmik FV15HP