I recently got a D800+ and I've found it is a fantastic sounding amp. Having upper and lower mids with semi-parametric EQ is really handy. I usually engage the bright switch. The deep switch is bit much most of the time, but it can be used to good effect in parts of songs.
I play with several bands. One has a really loud drummer and the other a really loud guitarist. Playing though 2 BBIIs it is enough for keeping up with drummer with no PA support, but it's almost running flat out. With only one cab, it was struggling to keep up with the loud guitar. It could, and sounded OK, but it was limiting towards the end of the night. (It's so nice when you get a chance to play with people who treat their backline as mointor only - same with drums if they are micd up. It's nice when someone plays the drums like a gent.)
The GK MB800 fusion, however, is an absolute monster. It seems to have limitless power. I've had it about half way up at a gig, before I realised it was a really weird sounding room and it was EQ and not volume that was need to hear my bass. That was rattling teeth and was uncomfortable to stand next to.
GK sounds great too, you have to dial in a fair bit of EQ to have much of an effect. I quite like the aggressiveness of the GK.
Both great sounding amps. The MB800 is waaaaay louder.