Brian, as I recall, the centre channel of 5.1 does not have to go very deep. Maybe just down to about 100Hz, but please verify. If that's the case, a Therm-8 in a scaled down cabinet might work. I'm picturing a cabinet with front baffle 24"w x 18"h, and perhaps 12"d to give you the volume to go down to 100Hz.If you think of the Therm-8 as a Studio-2 Tower with PSD2002 and midhorn, the centre channel I'm proposing would be the Studio-2 Bookshelf version of it.
Gar.