Substantially in agreement with you. Ideally you‘d have an integrated Server/Dac with an integrated analogue attenuator and a very high quality clock. Given the output voltage of normal dacs, I don‘t believe there is a point for a pre unless the power amp has unusually high input impedance. The point about the clock is very material: I have had very major benefit from using a LHY Audio Ocxo switch AND a 10m controlled Etherregen to clean up the Ethernet signal into the server as well as reclocking the actual DAC conversion.
My comment on the vagaries of USB relates both to noise from the contained 5V DC line and to ground level and RFI/EMI incursions. Many dacs to this day don‘t have galvanically isolated USB inputs (I use Intona Isolator and cables) and using dual USB cables with a separate 5v LPS again has yielded major benefita