No, I haven't heard a GAT, I would love to, but I also don't have $20K for a GAT S2.
The ET-6 is an ET-3 with WBT RCA jacks, other than that, it is virtually identical to an ET-3. This was confirmed to me by a friend that was in the market for a new CJ pre and called and spoke directly to Jeff, the principal owner at CJ, and asked what the difference between the ET3 and ET6 was. Jeff said the chassis for the ET6 is taller because the size of the WBT jacks mandated a taller chassis. I've heard the ET-3 and it was really very nice, but...it was not as good as my Pr17LS, and the CT-5 is clearly superior to the Pr17LS.
Not sure what the retail price of the First Sound is, but the ET7S2 would be the similar model I would assume - a lot closer to GAT than ET6.
personally I could not live with 2db volume steps and no remote

enjoy the new preamp.