Assuming you are listening at the same volume and aren't touching your software settings, it sounds like it's potentially an issue with clipping your DAC, which can happen if your media player is using EQ, bass boost, tone controls, or even if it allows gain beyond 0dB. Make sure the gain is properly set in your media player and turn everything else off.
It could also simply be a DAC defect, but I'd look to your software settings first.
Thanks for the timely response! (Your Mad Dogs are fantastic, by the way! Found out about them via r\headphones thread I ended up posting and the cans fit my preferences to a T) I turned off the high gain setting on the O2 amp and everything is back to normal. I'm using WinAmp for a player of choice and have the EQ/tone controls disabled. EQ/tone controls are also disabled in Windows.
If you are using the high gain setting with the O2 amp, there is a good chance that you might experience some clipping. Try the lower gain setting on the amp and see if you still get clipping.
I've actually cut a couple of resistors on my O2 so that my gain is 0dB and 2.5dB. I only use the 2.5dB setting when I connect my iPod via an LOD cable. For my sources with 2 or more Vrms, I use the 0dB gain setting.
I disabled the high gain setting and it cleared up all the clipping. Cans sound great again! (Is there a particular reason that a higher gain setting would cause clipping?) </newb questions>
Thanks for the responses!