Found some odd stuff and a fix:
Changing left/right balance (via Audio MIDI Setup in Utilities folder, or easier in System Prefs. > Sound) to either all left or all right channel completely resolved: treble tinniness gone, muffled sound gone, awful audible compression artifacts gone, etc.; plus all sound still came through both speakers and headphones which shouldn't happen if I'm running only all left/right - so something goofy was happening. On weird hunch I unplugged the audio splitter I have plugged into my Macbook audio-out port (I had headphones plugged into one output on it and speakers into the other; it's a Belkin splitter). All audio problems completely resolved once I removed the audio-out splitter (yes, I changed channels/balance back to 'normal' so I'm hearing my normal two-channel, balanced sound, both left and right).
@Mystic - do you have your audio output from Mac being split by anything? I've never had an issue w/ my audio splitter on three different Mac's in eight years, but something in either recent iTunes 8.0.x update or Quicktime is having a meltdown w/ my splitter so I'll be manually changing speakers/headphone connection when I switch between them. I'm curious if you're also using a splitter - hope this might help, as well.