The 10K input sensitivity of my ATC SCM19A actives was also an issue with the Sachs. Don would have needed to use (very good) Mundorf 3 uF silver oil cap instead of his favorite Polish .47 uF oil one for fit in the case, with $125 for the difference. Not an issue with the Supratek.
As you say, one difference is that the Supratek has a separate power supply, which means an umbilical. The one provided is not bad, but my experience is that the SQ effect of umbilicals and even 5v lines can typically be improved significantly (as a textbook engineer, Mick doesn’t believe that could be true). Ironically, I lucked into a cable maker who also doesn’t believe it, but nonetheless offered to make me one in a private deal with the Belden high quality copper with silver plating that he uses for internal wiring of the headphone amps he builds. The result was like taking the Chardonnay a good half step up the sound quality ladder. Revelation Audio also makes them commercially for the Supratek.
My only worry with Supratek is the cost of shipping were a tube socket to go bad.