Not sure if this is too late. I had the original ACT2, ACT2.2 and then the current GAT.
The ACT2 back then was the most neutral preamp I had ever owned. It was smooth, quiet, and with good detail, great micro dynamic, and big soundstage. Nothing special. Overall a very nice preamp.
The ACT2.2 (that I bought, not upgraded from the original) had more detail than the original. The ACT2.2 was a bit more dynamic and more transparent midrange. Transient attack was quicker and sharper. The original ACT2 was reticent and polite in comparison in the transient response department.
Now comes the GAT. Compared to both the ACT2 and ACT2.2, it is way more dynamic, huge soundstage, extremely extended and smooth highs, incredibly transparent across the whole spectrum, deeper and stronger bass, more body and texture for the instruments, the lowest noise floor of the three. Detail is quite close to the ACT2.2; but the detail is easier to notice with the GAT due to its ultra transparency. Jonathan Valin's review on the GAT was right on. I agreed with him.
To me, the dynamics, especially macro dynamics, and transient response are jaw dropping with the GAT. So far I can't find any fault with this great preamp.