If only some of your guitars are sounding "harsh" above the 10th fret, the first thing that comes to mind are the pickups and if the amp is dialed in for those pickups. The word "harsh" means different things to different people, so the solution kinda depends on how you define it. For the 800, be careful with the Middle and Treble settings. I would back off the mids first (4-7 typically works well with that amp), and then check your treble (I normally keep mine between 2.5-5 depending on the guitar).
The checkmate pack was the first Marshall cab I ever captured, so it's not a DynaCab with movable mics. The MRSH 412 M25 was a different capture session I did when I moved into my studio. The KW is my favorite Greenback cab, but that's just personal preference... some people prefer the KW and some prefer the MRSH M25.
I have an original Soldano 4x12. It's more compact than standard 4x12s, it's front loaded, and the stock speakers are Eminence Legends. To be honest, the cab doesn't sound very good at all. I've loaded it with different speakers and it never quite hits the spot no matter what I tried. I only capture cabs I enjoy playing through, but maybe I'll find a good one at some point.