One issue may be volume. It seems the HD570 is 120 ohms and has a sensitivity of 95 db/V. The ATH-M50 is more sensitive and has a much lower impedance. You may end up with the same problem as your Realtek onboard sound.It really depends on the UCA202 though. USB is a 5V source. At least in theory, 5V should be enough to drive the HD570 to loud volumes - it really depends on how the amp in the UCA202 is built.
Thanks a lot for the information. I decided to go with the NuForce uDAC-2.
I use UCA202 as DAC + headphone amp at the office. It's paired with Audio-Technica ATH-M50 and they work like a charm. Sound quality is just as good as you'd expect from essentially transparent device.
It's cheap and may be good for the price, but these devices are not made to be transparent. They are made to cover the basics. It has a relatively high output impedance, 50ohms, which will may cause frequency response issues. Typically these devices have audible tracking problems on the volume control. I'd be surpised if it didn't have an audible noise floor. That said, quality is not going to be a problem for the OP, who is concerned about volume and in all other respects it will be much better then typical integrated sound.
I suspect the maximum output voltage for the udac is not stated properly. They also state the RCA output is 2Vrms. How can the max output of the headphone out be 1.13 volts when the RCA outs are so much higher?