I had the opportunity to try both the Quilter BB800 and the Mesa D800+ side by side for a few days, and I have to say that I liked both amps quite a bit. The D800+ is capable of very deep lows, and of course has a more extensive EQ section, including an active treble control, which the Quilter lacks.
But the thing that sold me on the Quilter was that I consistently found it easier to quickly dial in the sound I wanted, bing bang boom. I could get a comparable tone with the D800+, but it took me a lot more fiddling around with the tone controls.
And this may seem funny, but I found I MUCH preferred the sloped front control layout of the Quilter (and those lovely large white semi chicken head style knobs). I quickly found that in a darker room, the angled front panel of the Quilter was still super easy to see and navigate, while the Mesa's straight vertical front panel was always in shadow, so it was difficult to see which knobs did what.
I did LOVE the connectivity potential of the D800+, with it's pre/post DI with ground lift, tuner out, and FX loop, but ultimately I ended up going for the Quilter. They are both GREAT amps, and they both are capable of producing excellent tone, so I think this kind of choice will tend to come down to each person's priorities and personal preference.