@soix Hi, I can understand your frustration.
Please tell me how I can clarify. I would like to keep seeing your inputs to my questions, even if it is not complementary. I value what you have to say.
I cannot explain why this XP-12 was a failure in my system. I did swap out other preamps such a couple of vintage APT/Holman ones. I had no problems with either. I even sent the XP-12 back to the factory. I live 30 mins from the Pass facility, so I hand carried the preamp to them and explained directly to the tech what I heard. He did find a problem with the older gain modules. He replaced them with modern ones and retested OK. However, when I put the XP-12 back in the system. the sibliance was still there, unlike all three of the other amps. So I just don't know why I had this experience with the XP-12. I did everything I could think of to make it work.
The suspicion that the 38s preamp is masking a different problem may have some traction. The problem with the XP-12 occurred with all inputs, two phono cartridges Koetsu RS and Shelter 901, the Paragon as Phono preamp and the SUT, one Technics SL-1200G TT, a re-aligned Sansui TU-X1 tuner, and the Bryston BDP-3/BDA-3 combo. I can't see how each of these could separately create the same sibilance.
I have looked at the Bryston BP-17 cubed before. Perhaps I should pick one up and give it a try.