Big fan of the Maverick TubeMagic D1+ here ... I got two of em.
I have the OPA627 op amps upgrade installed, and use a 1952 WE 2C51 or Tesla 6CC42, both of which play well with the tube set in the ST120.
The USB out uses the Tenor chipset, which to me is outstanding. Very smooth and refined. Tops out at 96/24, but I prefer that to the SPDIF optical or coaxial out which can do 192/24, but sounds a bit sterile to me.
Only issue with USB is that there can be a LOT of competition for computer resources. I had some mildly annoying dropouts and ticking on the latest HTPC build. Come to find out, it was my wireless keyboard and mouse. Solution was to install a USB expansion board that creates it's own channel and is hooked directly to the power supply instead of running through the standard bus. Zero noise and improved sound all around. Best $20 upgrade I've made.
The D2 works well too I'm told, but doesn't have the analog throughput of the D1+. I plug my turntable into that via an external stage so I can get some of that sweet toobie goodness on vinyl too.