I would take the SX-6300/R for sure if I couldn't have the SX-8300.
I think if you had a blind test with RBH vs Revel vs KEF with music that have bass only from 40Hz and up, you probably wouldn't know what you prefer more. The SX-6300 vs F206 vs R700 are about even IMO on the midrange and treble. But the SX-6300 will have a lot more bass than the F206 and R700 for sources that have below 40Hz. In 2.1 mode, definitely not much difference.
But the SX-8300 goes down to 25Hz, which is huge.
Personally I would take the RBH over the equivalent Revel and KEF if I were buying today.
With the (unconfirmed) active bi-amp of the passive subs of the SX-8300 & SX-6300, the difference will be even more pronounced IMO.
So whether the SX-6300 or SX-8300, I would do "double bass" active bi-amp which would help smooth out the bass response.