agree with your assessment. It seems though pretty damning that both on Ethernet and USB even on high end equipment you have to engage in extensive optimisation efforts before you get a decent result. As I said the single box solution can be seen as a capitulation.
USB yet again
For a few years I have had an Intona Isolator with Oyaide Continental 5S upstream and Intona Reference downstream connecting Streamer and DAC. Given the Strong benefit of filters on the upstream Ethernet connection I added a LHY Audio USB 3.0 purifier with a Grey Knights power cable. The tightening of the transfer and resultant SQ was remarkable despite having used superior cables before.
While USB remains a compromised transfer format, asynchronous USB is the only protocol synchronising the server’s and dac’s clocks unless both have master clock connections. AES/EBU may have better noise rejection but has imbedded clock signal,SPDif is outdated as well as speed constrained and I2S not standardised. Hopefully the industry comes up with a better solution. It is interesting that there seems to emerge a trend to combine server and dac: one wonders why?