The B810 is Eminence's own choice to put in SVT cabs. I've never used them so I can't comment on performance. But yeah, the CTS's in flatbacks, while not being very high power handling, sound awesome, and not just because they can be pushed, but because they have great tone that just plain works. The B810's should work in any 810 Ampeg cab, though, even the flatback.
Ampeg Portaflex Club #1