Hey, actually just made the jump from a MB500 to a Legacy 800 and have been very impressed.
There is something sonically different with the Legacy, and I couldn't tell you what that is. Maybe someone with a bit more experience can weigh in here. I guess the simple answer is it sounds like a classic GK amp, whereas the MB has a bit of that vibe, but not quite like the Legacy.
I never had a chance to play the original GK amps, my introduction to GK was the MB210. The legacy reminds me of the sounds that inspired to get into Gk (Duff/Flea), moreso than the MB did. That's not to say they don't share some characteristics.
I'm really stoked about my tone right now. It's definitely worth checking out in person if you can. It's tough to describe tone, but I'm very happy with the move to the Legacy.
I hope that helps!