Man, I think the issue with the highs is more of an issue with the MB than the cabinet. I've just spent the last 2 days with a MB500 head..... I really want to like this thing, because it would make the perfect backup, but the highs on it don't sound anything like my 700rbii. Maybe it's the pairing with the my GK 12's?..... but they sound so good with the 700rbii, that I'm not sure I'd like the MB with any horn loaded cabinet. It's almost as if it's like the 700 with the treble and presence all the way up.
Yes, the MB-500 is brighter than the 700RB-II through the Neo 112 cabs. For lack of a better term, the MB-500 doesn't sound as "natural" as the RB head with everything set flat, especially in the highs. It's not much trouble to dial out the brightness though, and I still think it sounds great. The footswitchable boost channel is awesome. Tone-wise, I do prefer the 700RB-II, but the weight saving of the MB head is a huge bonus and I expect to use it most of the time.
I've seen the Harke cab at Sam Ash. I didn't have time to try it, but two things stood out: really small footprint, and switching between 4 and 8 ohms. The latter is a wonderful feature IMO, especially for use with smaller heads like the G-K MB-200, and combos like my Roland Cube 100 that has its extension speaker output wired in series with the internal speaker, and thus requires a 4 ohm extension cab for full power. I'll test it with the Roland one of these days.