Hmmm... New motherboard, Windows 7, low audio volume...

...after thinking too much about solving this issue with my fancy 'everything AND the kitchen sink" motherboard - it turns out that listening with headphones when your HD audio thinks you're plugged into a 7.1 surround sound system (no idea why it defaulted to this) results in really low 'max volume.'

Settings it to 'stereo' nearly blew my head off (since I was playing some Buckethead on full blast...)