I am using the xduuo for a couple of months, but I have tried listening without it in the chain. I cannot say that with the tubes the added the difference was night and day - just a little more depth and less edges, not the holographic holy grail of tubes I assume is there when your power amp is using tubes. The Xduuo is just a buffer in this case, albeit OTL. Of course with the E30 II adding the tube pre I got a less black background but I did't like it with or without the xduuo. Ignoring the harshness, as this also depends for me on the speakers and the amp, the sound was just too claustrophobic and missing top extension.Of course that's because you're used to hear the harmonic distortion of the tube and your system.
And yeah, there is no "synergy" in the objectivist world like in ASR.
Anyway, you are right about this being ASR, but in the end I have Amir's measurements of the E30 II as they are part of the in-box materials they send from Topping. I look at them and by all accounts this should be a stellar dac, yet it isn't pleasant to listen to.