Oops, forgot about the requirement for USB input. I already knew that any Assemblage DAC would have to be bought used. I wonder how hard it would be to add a USB input to one and whether it would be worth it. Time to use your product to find USB dacs.
Ok, here's a quick (and definitely not exhaustive) list of USB DACS to get started:
1. Wavelength Brick - retails for $1750. You might find one used for $1000 or less.
2. Wavelength Proton - coming soon. Will retail for $900.
3. Stello DA100 - $695
4. Apogee Mini-DAC - $895
Note that I've not heard any of these DACs and, to the best of my knowledge) all are single-ended.