Best advice, get rid of the BTB. Their motto is big body, big tone, so that means big weight. My old 676 was probably in the 12 pound range. That thing was rough for an hour set, full nights were nothing short of awful.
Class D heads have no problems with lows, I have never experienced any kind of shortage if anything I turn my lows down at times.
Carvin LB76 / Dingwall ABZ ! Support Local !
MarkBass SD 800
Epifani UL2-310 / fEARful 1212/6
! Rocking against all gods !