OK, turned it on, selected #9 Over Chorused Clean, sounded fine lots of chorus. Plugged in MXC, MP-1 went to #1 so normal. Pressed 9 on MXC about 10-15 seconds later MP-1 changed to #9, chorus still fine, no funky business. Pressed 8 on MXC, MP-1 went to #4 after about 10-15 seconds. Turned everything off except MP-1, so MXC still plugged in but no power to it. MP-1 stayed at #4 for a while (while I type this post), I just turned around and checked it, MP-1 now changed to #1 (MXC still plugged in but powered down). Pressed MP-1 Up button, display flickers between #1 and #2 while the button is pressed then back to 1 when released, if I hold the Up button on for a while, it flickers between #1 and #2 a number of times after I release the button (like command is in a buffer?). Pressed MP-1 #3 button, display flashes 3 while pressed then back to #1 when button released (but doesn't flicker between #1 and #3 like Up button). Unplugged MXC and cycle MP-1 power, now front panel buttons work as expected. Plugged the MXC 7 pin cable back in but no phantom power, MP-1 continues to work properly. So powering up MXC when connected starts the funny business which doesn't go away until MP-1 rebooted.