I gigged with a Shuttle 6 for a good few years and have recently converted to a GK Fusion 500.
I was perfectly happy with the Shuttle, it was always loud enough with plenty of head room to spare. I was always able to eq it to the room and I could always be heard, loud and clear. It was lovely and articulate. It could do very clean or down and gritty very convincingly.
I had no reason to change it, so why did I? I think really, it was just down to a combination of fancying a change, spending too much time on BC and hearing good things said about the GK.
Having said that, I have no regrets about buying the GK, it is a brilliant amp. The two channels has been a very useful addition and the eq is extremely user friendly. The valve pre amp is great, does generate a fair bit of heat though, so the powerful fan was a bit of a surprise the first time I heard it. Obviously, not a problem in a gigging situation but loud at lounge level.
My only concern initially was the lack of a clipping light on the GK but in reality, it's not a problem.
I would be perfectly happy gigging with either of these amps.