Thought I'd give this thread a bump as I've had the GS-1 in my system for a few days now and I'm very happy with the results. It sounds superb hooked up to the Duet, whether feeding my speakers or headphones. I don't find it at all dry or cold or solid state-y as I thought I might coming from a tube amp. The only change so far is that I'm listening to my 650s more again, as I think they might shade the 600s in this particular combo. It's nice to hear those extra bass notes when they are well controlled.
Must admit, I pulled the trigger on this amp in a rush of blood and then was a little bummed when Ryan's Zana came up shortly afterward, because at the back of my mind I half expected to still end up there anyway. Now, though, I have to say it's really hard to honestly see myself needing to step up much from this level of performance. Such an elegant little thing, too.
I'll stop gushing now.