4-string

I happen to own both these amps. Both are great, can't go much wrong with either IMO. RH750 is the more versatile of the two, amazing EQ and features: 3 memory presets (which includes preset level, brilliant!), built in tuner, good compressor, tube tone, universal volt...just killer. You dial in pretty much anything but a big, wide, open uncompressed low end. Cue MB800 - it has exactly the uncompressed big lows the RH750 lacks, and is also very bright out of the box. Great GK eq, channel A and B which are for different gain structure only, eq remains the same. RH750 has lots of warmth/midrange distortion built in, but despite its warm character it never lacks clarity to my ear. The MB800 has that famous GK aggressivenes in the upper mids. Two very different, but great amps. I prefer the MB800 with my Barefaced Big Twin because that cab has no restrictions in the low end it seems. That particular cab and the RH750 works well enough, but with a wider more open head like the MB800 the cab just sings. With more traditional cabs though the RH750 is amazing, so I keep that with the RS210/RS212 stack. As for being loud I was very surprised that the RH750 seems to keep up with the MB800 volume wise - both are silly loud amps. In the RH750's case this is of course on the cost of a more compressed low end, but still those 236 watts are quite impressive. If you have one of the Big series cabs, or a fEARful or some of that ilk, I'd recommend the GK. With more traditionally tuned cabs my nod goes to the RH750, especially for what you are describing in your initial post. Unlike many people I am not too crazy about slap through GKs, I don't like that much distorted and aggressive upper mids for that. This can of course be tamed some by eq, but the character is still there. IMO!

I too have the RH750/RS210/RS212 setup, amazing rig! Highly recommended.

Recently got a Big Twin T myself, impressive cab! I like to hear all my mistakes and the flaws in my technique, as they all tend to come through well in the PA. Everything is revealed with this cab, which in my opinion is a good thing. There is more high mids than I have ever heard in a cab before, no need to boost them anymore... Also, the lows are just impressive, no compression at all. The biggest advantage for me though is the weight - it is just unbelievably light, and having struggled with a bad back for years that is worth A LOT after a three set gig. Granted, there are other light weight cabs out there, but I doubt there are many (or any!) that can match the sound quality and output of the Big Twin.

GK MB series. There really should be an attenuator for the tweeter, but nothing the tone control can't tame. Sounds great IMO.

Power hungry Schroeders? Where do that come from? Yes, they can handle lots of power, but are very efficient cabs as well. I would definitely go for an 8 Ohms cab with the LM800, good to know you can add a second cab later.

I used to gig with an ABM EVOII 500 2x10 combo, which I used together with an Eden 1x18. Sounded great, but not as good as my current MK-500 / Schroeder 610L setup.

I've always wanted a Funkyzer knob.