It seems some like them and some don’t, or just prefer the sound of a more complete phonostage.
Dear friend: You are rigth but the question of your thread states: SUT or not SUT.
Other than the electrical match/mistmatch issues with SUTs for me the main subject is the overall quality of the carrtridge signal amplified and this depends only of the rigth and precise design of the active high gain phonolinepreamp or the SUT quality overall design.
First than all the the better and best active gain device for a MC cartridge are bipolars ones and for MM cartridges are FETs. Several active high gain phono stages use FETs for MC cartridges due that the handle and design is a lot more easy that to deal with " savage "bipolar where we need matched pairs to start with.
You own an not easy to digest electronic items combination in your system using SS/tube units. The XP25 is a good phono stage but not the best out there. Pass is way better with amplifiers that with phono stages and not today but from he was an owns Treshold and used bipolar output devices in the top of the line Stasis e pure class A amplifiers. Lovely design and quality level performance:
https://www.hi-fi.ru/upload/iblock/1a2/1a27110b8c03c44cc905cc7defc8a5af.JPG
A SUT has inherent disadvantages because normally you need additional IC cables with additional input/output connectors and solder joints where the cartridge signal must pass through and all these additional steps makes signal degradation no matters what the other disadvantage is about its frequency range limitations.
A good active high gain phono stage has none of those disadvantages and several advantages, again with the rigth design using the rigth gain devices.
My phonolinepreamp performs really good with any cartridge but I own and owned several SUTs because are " atractive " alternative, I can say a different alternative not better one. It can't be but good alternative.
I own and owned several SUTs from very humble as the Denon 340 to glamorous Kondo and the best I experienced in my system and several other systems is the Denon AU-1000 that rigth now I'm listening through:
https://www.denon.jp/ja-jp/museum/products/au1000.html
Look the frequency range: 5hz to 200khz flat. Try that the SUT you choose has this kind of frequency range response, extremely important for the quality level performance.
SUT or not SUT: you has to try. I don't like that Thoress equalizers kind of design but this is up to each owner.
Yes, my opinion is that not SUT combination can beats a good active SS high designs but the issue is exactly that: good designs and this is not easy to find out, so the SUT is good alternative and as you said some are seduced by.
Regards and enjoy the MUSIC NOT DISTORTIONS.