The slim and stage 2 have a separate processor for handling midi. There is a firmware to send that processor that is different than the OS. They call it the “USB firmware.” On the stage 2 and slim the 2nd processor handles all midi, both USB and DIN. Even though, the name might suggest otherwise. The tribute and stage 1 process midi with the same processor as the sound engine. From the moog board, posted by Amos (moog’s OS programmer),

"but first, here’s the USB firmware sysex file for Little Phatty:
[ltr]http://seamslikereality.com/moog/LP_USB_v33.zip[/ltr]

and here’s the USB firmware sysex for Slim Phatty:
[ltr]http://seamslikereality.com/moog/SLIM_USB_v34.zip[/ltr]

try sending that file via USB to the Phatty… it might fix the problem.
seems to work better from a PC than from a Mac, in my experience so far."