I read your posts with interest. I use all tubes (Rogue) but for the Musetec DAC. I am surprised by your comment that age brings about sensitivity to higher frequencies, as I’ve always thought the opposite. In any event, some tube rolling can tone things down. Like dbb, my interest is in classical music, IMO the most difficult to reproduce. And, as in analog, detail matters.
I continue to compare digital to my analog (still all tubes) and I find it comparable. Sometimes, but not always, the same recordings sound eerily near each other, but for the usual analog sounds.
I find the Musetec to be a truth machine and to be extremely responsive to what comes before it in the digital chain. I use an SOtM unit as a DLNA device with a very simple laptop running JRiver. As I wrote in another place, even adding an $18 ethernet switch and having my hard disk server bypass the router yielded a surprising SQ benefit. Is an audiophile switch in my future? I don’t know.
I have also found it quite responsive to the USB cable, and I do recommend that the Musetec is optimized for USB. But it needn’t be an expensive cable. The DAC seems so cleanly extended at the frequency extremes that a simple solid core copper is doing better for me than expensive silver wires.
That being said, as a truth machine it will reveal the truth of your software, sometimes as gloriously recorded; sometimes not so much. Did we not go through exactly the same thing in analog?
it seems that you have a great deal of experience with a variety of DACs and have not yet settled on a Sigma-Delta v. R2R. I don’t yet know whether you are sufficiently convinced about the Musetec, but the current sale price at Shenzhen is attractive and they offer a 30 day No Reason Return. While you would probably have to cover return shipping, that is likely to be less, for example, than the 5% restocking fee plus 2 way postage for a return of a Schiit Yggdrasil to California. Perhaps that adds some perspective.
Good luck to you whichever direction you go.