I've tried so hard to like the GK model, but it just doesn't work for me. I think I'll just have to admit I don't like the sound of GK amps! I'm glad to know other people can make it work.
RE: the brightness of the Ampeg captures - 100% agree, it sounds to me really high in the upper registers. That said, when I've put them in the mix they sound fantastic; I ran a gig a few weeks back and had all the bands bassists going into the QC (I gave them the option to go DI from my TC rig, but I think they all wanted to have a play with the QC). With a patch consisting of a bit of compression and one of the Ampeg captures, I actually ended up boosting the high mids even more for some of the heavier numbers; it sounded absolutely monstrous! I even had a couple of people come up and ask me what the bass players were using because it sounded amazing!