I have a GT10, but I only play through headphones since my $150 Behringer broke. It sounded great through that. It basically sucks with headphones and going straight to the computer with USB. It sounds very harsh and digital, but I didn't get that playing it through the amp I'm considering getting the Carvin tube poweramp and 2 Basson 112s for stereo. Would that setup improve my sound by much or am I better off just buying a tube amp?
It's really meant to be plugged into a guitar amp. If you're using the GT10 extensively, just about any SS amp or PA system would do. Preferably, you could use the gain channel of a tube amp for your distortion and use effects and modulation from the GT10 for the best tone.

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if you want a tube power amp I think atomic amp's are meant for multi-effect preamps. And, as stated above, any SS or PA would be good. A really clean clear amp like a roland jazz chorus would work best though with multi-fx.
I just looked at the JC-120, it looks great, and it's the best value. Most of my patches are stereo with a bit of delay between the channels. The JC-120 is stereo and it has 2 input jacks, but do they work at the same time?