I'm not sure if that's a good idea. According to NwAvGuy, peaks in audio at standard listening volumes can be as high as 110 dB for which you will need ~2.5V from amp with HD 600 or 650. But O2 set to 1x gain with ODAC would be able to deliver only 2V.
I think it's just easier to lower volume in PC.. very probably you won't here any difference in terms of audio quality (Once I made a test with ODAC set to 5% at 16 bit and O2 at full volume, and I couldn't tell the difference with HD 600 vs ODAC at 100% volume @24 bit)
Edited by Dano91 - 4/26/13 at 11:08am