A great question and very dependent on a large number of variables, so difficult to say what is "better". I have tried various SUT's and head amps into my Aesthetix IO Sig, primarily to cut down on the tube rush at high gain (you remove some tubes for lower gain in this model).
SUT's generally add a level of dynamic, but can get brittle at the top end very much as Atmasphere describes. With a Koetsu, this can make it sound more alive, but I also agree there is a "ringing" effect that obscures detail. Having said that, in experimenting with loading my beautiful Verity Z, SUT beats, IO alone, Zyx pre pre and Accuphase C-17 and Pathos phono alone.
On my ZYX UNIverse, the ZYX pre beats IO alone, and best is the C-17. In general, my C-17 / IO gives better results, texture, soundstage, depth and believability, than any other combination. However ..... it took a lot of experimenting with IC's between head amp and phono before I got to that point and some cables made this combo the most ordinary sounding of all permutations.
So, I conclude, there is no specific recipe, just lots of trial and error to find the sound that suites your needs and I would argue strongly that there is no quantitative evidence that clearly indicates a preferential choice, notwithstanding personal preferences already noted in this thread by experienced audiophiles, with whom I would never argue with!
Steve
