I suggested the GS-X though I have not heard of it, because of what the OP requested "A sense of crystalline clarity is number one on
the list of wants." I guess you have heard the DT880 on the Stratus or any EC amps? How do the combination sounds? why is the DT880 better with those amps than the GS-X?
"A sense of crystalline clarity". My personal preference is not for transistor headphone amps, although I do like some of those I have heard on speaker amps. The offerings by Crimson Electronics have impressed me tremendously for their transparency. Transistor amps, to my ears and brain, can be crystal clear, but still can be quite forward. In transistor headphone amps, I find both to be true, but I have not sampled that many, but for those i have, not for long because they are so forward.
Keep in mind, my preferences in speaker and headphone amps favor those that use directly heated, single-ended triodes, like the 45, 2A3, 51, 300B and quite a few others.. Woo Audio and Eddie Current make a number of such amps. Donald North makes just the Stratus with the 2A3; however, he also makes an amp called the Sonnett that is an SET, but uses tubes that are not directly heated. I have not heard the Sonnett.
I have never heard the GS-X, but I have heard a number of Woo and Eddie Current amps. Overall, I prefer the Stratus.
Take your time. Try to make a meet where you can sample some of the different varieties of quality SS and tube amps.