I use an old Marshall JMP Superbass head with it's 4x12 Greenback driver cab, i love it. when you crank it up you get a nice tube overdrive, it's not for people who loves sparkling clear and defined sound. It's pretty loud too, the drummer can't cover you. Before i used the JMC Superbass with the 400w 4x12 cab, very good too, a bit clearer and very LOUD! I've never played GK so i can't do a comparison, but i don't think they have much to share soundwise. You may want to consider the actual VBA amplification from Marshall, should be very nice and powerfull.
tschüß