Ked according to Mark at Malvern Audio Research, my Big 7 had a clever valve microphony minimisation circuit.
Your GG1 (and Rob's) never had this circuit.
When my DAC went back to Lucasz they ripped this circuit out and replaced it with a circuit like your GG. My DAC is now as susceptible to microphony as any GG1 and my guess would be later GG DACs too.
Whilst this circuit sounds better, it is far more susceptible to microphony.
Possible. I also think it happens with some preamps. Since you have changed to lux, I don't think I will now face it at yours. Heihei has tried with Soulution he didn't have any problems, and I will soon, to check if mine has microphony with Soulution pre.