The BB power figures are maximum clean, or else fudged which I wouldn't expect from Mr Quilter. Reports are Elf is an honest rating so the two should be equal at the 200w setting on the BB.Thanks for your help. English is not my native language, plus you're going waaaay too much in deep with technical stuff I don't understand very well. This is what I know:
My amp gets 800W at 4 ohm, 400W at 8 ohm. My only cab is a SM (8 ohm), so I have 400W basically. I just want to know if an Elf+single SM can deliver enough volume to play at rehearsals and maybe small gigs.
If you can't get loud enough before distortion with the BB on 200 then Elf is not going to cut it in a 1:1 backup role.
With PA support it should probably get by fine and in lower volume situations also handle the jandal.