Not relevant, but this...

when I toggled the Hardware Flow Control setting with the empeg connected, the empeg promptly went into shutdown mode

...reminded me of something.

I was playing with the Windows Media SDK yesterday. One of MS' sample applications reproducibly rebooted my car player. If I disconnected the serial cable, all was well.

The sample application was supposed to be enumerating sound card devices, so I don't know what it was doing playing with my serial port...