I've been A/Bing the two amps. They both sound great. Based on my limited time with them, I've noticed the GK is a little more punchy and grindy sounding, something that I've always heard from their amps. The RH750 is a little smoother and old school sounding by comparison.
One of the main reasons I went looking for another amp was that I started noticing that the RH750 had a bit of fuzz around the low end notes. I remember reading something KJung said one time about it being fuzzy on the low notes or something like that. At the time, I didn't understand what he meant, but after spending more time with the RH750, I think I do now. It seems to be when I am trying to do quick, punchy, staccato riffs the response is less articulate than I would like. The MB800 does better for that it seems.
I've got another rehearsal with it tonight and planning on using the MB800 on the gig Friday night. Time will tell, but I am digging the sound slightly more so far. I can tell I miss the built in tuner already though