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.