I also like my Gk 700rbII that i got at a flea market for 75 bucks.
Damn, that's a killer deal!! I lucked out and got my 800RB for $400 shipped off the classifieds here, I know they sell for cheaper, but mine is a later one that's in excellent condition, probably the cleanest I've seen for used ones, the former owner must've taken exceptional care of it!!!
After reading through so far, I'm not surprised about the love GK gets, I never gave them a chance because I played through a GK 1001 for 6 months and never bonded with it, I just found it too clean/sterile for my tastes....
About 3/4 months ago I finally tried out an old beat up 800 and I was in love within minutes!!
This last March I had an audition with a power metal band and played through the 400RB that is the backline at this local rehearsal place.... No wonder Duff used a 400 for "Appetite..."!! Through 1 Genz Benz 4/10 it kept up with 2 guitarist and a loud drummer with minimal effort or volume and sounded amazing unlike certain other SS amps I had in the past, though I will point out I've only owned 2 tube amps(SVTII non-pro and still have a Fender PS300)but the 800 truly has the same"liveliness/character" that reminds me of tubes!!;
The only difference I could hear between the 400 versus 800 was the increased treble and headroom with the 800, where the 400 is warmer and the 400 gets grindy much easier....
Both sound lively and have a nice juicy tone, I really dig the mid contour and "hi boost" on it, it remind me of the ultra hi/low switches on an Ampeg..... also seem the compress heavy handed fingerstyle much like the Ampeg I once had!!
So in the next few years I might get possibly a 400 as well or another 800 as a backup(not that id need another, but just in case

)
The RB series made me a believer!!