My O2/ODAC has no issues driving my K702 with any song I have (even very quiet high DR tracks). I have the Mayflower O2 amp which I believe has 2.5x gain for the low setting and 6.5x or so gain for the high setting. On high gain with max PC volume (using the ODAC) there is a lot of clipping which is to be expected, since the high gain setting is actually only for using the line in, and not the ODAC. The high gain is for quieter sources, not harder to drive headphones.
8/20/14 at 11:02pm