Not having read the entire thread, I'm just posting in reply to the subject line...
My vote, if I may, would be for MC9 to have it's own volume control, as Windows Media Player does, independent from any other volume controls in the Windows standard mixer.
The ability to map MC9's volume control to my Streamzap remote would be a major boon, especially if it could be done with a minimum of end-user configuration.
MC9 has it's own pre-amp and can adjust its own volume based on Replay Gain, Equalizer settings, and a variety of other causes in the DSP studio. I believe that if MC9 can do this, it'd be worth Matt & company's time to tie it to the volume control slider. MC9 with its own independent volume control would allow all users, not just audiophiles, to better and more precisely control their overall playback volume.
(and aye, it's shocking to hear the logon/logoff music sometimes, depending on what I've been playing in MC lately, hehe)