Well mine arrived today and I've had about an hour to compare it to my subway d800. This was playing through my 15/6/1 4 ohm cab.
Headroom goes to the bb800 to get the same volume with the d800 it will start clipping.
The tone on the bb800 is exactly what you put into it it truly sounds like your bass . The d800 has more of a thicker rounded bottom end to it ,it just seems fuller.you can get there with the bb800 you just have to crank up on the deep circuit.i typically play an American p5 or my esp Rocco prestia 5 . Both amps are great but when you add a pedal things get more interesting I use a zoom ms60b with just a couple of presets one of them thickens the p bass up a bit and the other kinda leaves the esp where it is just adds a little compression to both patches.If I were going to be using a pedal most of the time the bb800 is the winner for me it seems to give the feeling that you have a tube amp behind you ( it's hard to explain) on the other hand using the same patch with the mesa I have to turn off the deep switch it's really freaking close . Although the mesa should sound warmer it just doesn't seem to have the heft on the lower range notes that the bb800 . It's really all subjective apples to oranges . For the shear headroom the bb800 will cover most my single cab gigs I will probably keep the mesa for two cab gigs where I need to be able to run both an 8 and 4 ohm cab together . If the quilter would do that I would probably let the mesa go. IMHO will have to gig them back to back to be sure.