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?