I second Micro's observations...my own:
- The extremes of the GAT2 are more finely polished and articulated. The ACT 2 was perhaps articulate in the upper treble but not nearly as refined nor as tonally pure as the GAT series (and I am told the original ART series though i have never heard those).
- On the bass end, i suspect capacitance, power-related upgrades have improved its punching ability quite noticeably. It is a continuation of an extreme purity refinement that the GAT finally started to get right as a blending of the various powers of the original ACT 2 and the ART series from my discussions with various fellow owners and reviewers
- The system is also much, much quieter once the GAT 2 went in. The GAT did also have more trouble not eating through tubes, and so far I have not had this problem...and having swapped for Mullards...cannot say that going back to stock tubes does not truly satisfy as well. (i could not say this about the stock tubes of the original GAT which went noisy within a few months...consistently. Like many owners including XV-1, i went to Telefunkens).
- It seems to have found a following of owners who have 'set it and forgotten it'...i am one of them. I long ago stopped thinking about preamps.
The only other comment i would make is about Roy Gregory who has kindly offered me great and very well considered advice for nearly 10 years...very generous with his time and experience. I do think from his shoot-out review that the VTL brand is something for which he has a deep affinity. He has said the Siegfried II has no peers (at least that is how i interpret his review of it). Since he loves the Siegfried II and is also a reviewer who needs both SE and XLR, etc, etc...I suspect that combination probably does put the VTL 7.5III at the top of his personal list. Not that it matters...we all have our favorites. For me, the CJ GAT 2 is it. If i ever were to consider the Siegfried II...that is probably the only way i might swap in the 7.5III to see what happens. But with the Gryphon Colosseum...i am not sure i see that happening either.