Finished building my Project Sunrise II last night (0221) and I want to thank geetarman49 and others for the recommendation and share my perception that the Project Sunrise II, in either kit form at $199 or built at $244, is clearly one of very few extraordinary values in audio.
I run flac USB digital out of Samsung S3 into iBasso D10, line out into the Sunrise II then, stock cable into my basic DRV#3 mod. The Sunrise has a much more power than needed to drive the headphones and I can't describe how thrilled I am with the amp. So far listened mainly to Pink Floyd.
Now I'm in the hunt for a great value in a DAC (<$300) that will make the setup even better.
If all you need is a pure USB DAC (and one that works with the S3), the ODAC is hard to beat. It has a very detailed and transparent sound and might do well with a tube amp. Reviews are still coming in, but Schiit just released the Modi which is only $99 and looks really nice. Early impressions are good. I have one on the way and can compare to the ODAC. Another option is the Nuforce HDP which you can get refurbished for right around $300 now, which also adds a headphone amp, preamp, and more digital inputs. Even if you don't need the other options, its DAC is quite good and a bit more refined than the ODAC.