Your requirement that the DAC can run off USB power disqualifies the oval MS II+. Although technically it can, I experienced intermittent static, I'm guessing depending on the load on that USB port. Using a powered USB hub fixed this. I used USBView to see if I could figure out what was up and it seems I've got something else hooked up to the USB 2.0 port I was using (I think it was the laptop touchpad, can't remember 100%). The oval MS II+ draws 400 mA from USB, which is more than the MS Pro (350 mA), and I think the max USB can give is 500 mA. I guess it's still portable if you don't mind carrying around a powered USB hub.
Sidenote, too, WASAPI doesn't work with it. I was getting periodic audio drop out, so I tried increasing the buffer in foobar, which helped, but I couldn't set the buffer high enough to totally prevent it from happening (foobar gave me an error message past 2000 ms, I forget what it said exactly). Dunno if that would be the case with all async USB DACs. Also, it's a little hotter than Redbook (2.25 V), haven't had any problems with clipping as far as I can tell, but thought I should mention it.
Those are pretty much my low points for the MS II+ and, other than that, I like it a lot.