Ok, this is a weird one. I have a Mackie MCU and 2 XT units. One of the XTs works perfectly well on my laptop. But my Mac Pro will NOT receive MIDI from fader #5. I've tried multiple cables, interfaces, etc. It is completely repeatable both in Logic and the Audio MIDI Setup application. The laptop is 10.6.8 and the tower is 10.7.5, but these are the old MIDI versions, not the USB, so I don't think OS should matter. Does anyone have a clue? The repair shop doesn't.