I'm in the middle of breaking in a pair of the A-S monos right now, and I am loving their sound thus far. I considered trying the AGD Duet monos instead, but I was scared off by the high price - especially for a product that I've never heard before in any audio system. I'm using a tube/hybrid preamp (a PS Audio BHK pre) that is sounding great with the A-S monos. It looks like you are also enjoying the combination of a tube preamp with a GanFET power amp.
Best of luck as you continue to refine your audio system!