I just took delivery of a GK Fusion 550 and I must say I am quite impressed. Sounds killer through the Bag End stack as you would expect. I wasn't able to find many videos of the amp being used in a rock oriented setting, so I shot one to hopefully give folks an idea of what the amp sounds like when used in this manner, and how much OD is available. With everything at noon, it has a very thick, meaty, clear, growly tone - it actually reminds me a lot of my own preamp. The video doesn't really capture the girth due to room nodes, but it should give you a basic idea. I was playing around a bit more after I shot it and came across a killer, sinister, grindy tone with gain at 11, bass at 10, both mids around 2, and treble at noon.
I'm not an earlier adopter, and hopefully all the bugs have been worked out with the knobs. I could do with out them, but assuming they don't freak out, I do kind of like the way it switches tones gradually rather than the more abrupt channel switching.
Cheers,
JR
YouTube - ‪GK Fusion 550.MOV‬‏