If warmth is a priority, IMO you should go for a Fusion variant or a 700RB-II. I have a 700RB-II and an MB-500, and the RB generally sounds warmer and more substantial. I can get into the RB's neighborhood with the MB-500 by cutting the highs and high mids to about 9:00. I cut the bass a little too. They have a similar presence/feel in a band mix, at least at the modest gigs I play, at which I don't push either one that hard. As a result, I use the MB-500 most of the time for convenience, but purely for tone the 700RB-II is superior, and you would likely notice the difference more at the higher volumes you apparently use.
Thanks for your input. I do eventually want two GK heads - one new and one used. My plan is a new Fusion and a used 700RB-II or possibly 1001RB-II which can be picked up pretty reasonably in the classifieds here on TB.
If I go for the Fusion 550 I'd have to wait another month or two to save up for a used RB. If I go for the MB Fusion 500 I could use that extra money saved and buy a used RB sooner.
I've been burnt (twice) in the past by only having one amp to depend on and had to run direct into the PA at a gig. Didn't care for that at all...
