Was playing around with my Keeley Rat into the GH50L today, I think I prefer to get the extra gain from a pedal rather than using the amps in built extra gain stage. I'm running the amp with preamp gain on 10 and the Rat infront on phat rat mode, level on full, filter all the way up and distortion just above 9 o clock. It seems much tighter, clearer, less fizzy, more defined and seems to have more harmonics. More of a "3D" sound and seems more dynamic.
I may still go with a tubescreamer though as the Rat doesnt have the mids or general beef of a tubescreamer, its more of a fuzz than an overdrive really. I think with a good overdrive I'll be able to get much better hard rock and metal tones. New guitar wont be shipped till wednesday or thursday next week :(