I just got rid of my MB Fusion as I liked how my M9 sounded a bit better. A BIT better.
The MB800 has more headroom than the MB Fusion though.
I still think the M9 is crazy loud and definately has more headroom than the MB800.... but IMO, it really should come down to tone, price and maybe weight.
The M9 in Canada is $1599 and the MB800 is $790. Is the M9 twice as good as the MB800? Not IMO. Is the M9 with more than 2x the price of the MB800? Not IMO. Also, weight is a consideration... The M9 isn't crazy heavy but it is a full 2U rack... and a nice Gator/SKB case is over $100 too. A MB800 will fit in my gig bag, the M9 obviously won't.
I DID get a great deal on my new M9 at L&M as the used one I bought died and they replaced it with a new one before the price went up from $1299.... if it did it again I would likely stick with Mesa, but the soon to be released MB Fusion 800 might just be the one.
Whew...
Lastly, Mesa has a 5 year transferrable warranty. No questions asked thru L&M and they take care of everything including shipping charges back to Mesa.
Very lastly, The GK MB series sounds a lot IMO like the M6/M9 tonewise. Try them side by side, I bet you will be suprised.
Good luck, but both are great amps!