Well, it's a USB DAC that I like and I'd prefer not to start talking about how I'm "having trouble" with it unless that's a crucial question here. I'd really like to just focus on the general question of volume control on iTunes/Macs.
It's a relevant question, since it's a very unusual problem. I've never heard of a USB DAC getting "overloaded" by the signal form iTunes. I thought that lowering the level within iTunes was a compromise in the dynamics of the music, and that it is best to leave the volume at max and do any volume controlling in the analog stage. I always leave the volume maxed in two systems and have never had a problem. I'd also be curious to hear which DAC is yielding such a problem.