I play a 700rbII, and the rb800..... I have a buddy here who has the mb500, and I've gigged it a couple of times. Seriously, I can't stand the little thing. I really wanted to like it, 'cuz the form factor makes it a perfect backup. I picked up the 800 as a backup/rehearsal head instead. Heck, I'd rather play a bl600 than the mb500. I can't quite put my finger on it, but the mb500 is missing something, and the high end is quite "brittle" sounding..... not quite as "airy" on top as the 800 or 700.
I guess what I'm saying here is that if you're really into the rb800's tone, then I don't think you'll dig the mb500.
The 700rbII is perfectly capable of copping the 800's tone. Some will say you can't, but I don't believe that. I've a/b'd mine extensively, and I think what people hear is the growl developing at lower volumes on the 800. At stage volume, the 700rbII gets completely "growly". Having the variable presence and contour controls makes it a bit more versatile, and it's got a bit more power.