Subwoofer: room 16 x 18.5....

I don't believe a 8" would work either.

For music, look for either a sealed or passive radiator design. The Velodyne SPL series and higher end Polk subs are great for music.

Bob,
How has the SVS SB12-Plus been working for you? Do you think it compares well with some of the 10" subs in the $2000. range? (e.g. JL Audio f:110, REL, etc.)

While floor area is important, the height of ceiling also plays a part. Room size in ft3.
Go to the HSU Research site and look at there room size chart.
That'll give you some idea of sub to room relations.
In any event, 8" is too small a sub, except possibly at low levels?

Measurements of subs, while a good thing doesn't cut it in a real room.
Google:: 'room mode calculator' and look for nodes /peaks and resonant frequencies below 300hz. they're the worst.
Also, Find the Harman White paper on subs. The bottom line recommendation is that 2 is MUCH better than 1.
In MY opinion, for a budget of over some amount, for me, about 1500$ or 1600$, it is actually better to have 2 subs in an asymetrical setup to reduce room nodes, both plus and minus. A pair of 800$ HSU research VTF3's would almost certainly be a match for ANY equivalent value of another single sub.
In my case, with a completely wacko, room of no less than 8 sides and an off-center vault ceiling, I make do with a lesser single unit. I'm not a crawl-around-the-room with an SPL meter playing test tones kind of guy, but I can detect no boom or suckout....except in my den where there seems to be a midbass peak which exagerates bass drum. I guess my den my be acting as a helmholtz resonator.

Seriously, check out my Google recommendtaions and have a fun read....shouldn't take more than an hour git-2-go.