I have a MacBook Pro 2.8 GHz Intel Core i7, 8Gb RAM, OS X 10.8.5. Prior to upgrading to the v1.2 M20d firmware, I was able to stream audio to and from the M20d over the USB cable without any problems. In addition to upgrading the M20d, I've also downloaded the v1.23 drivers from the Line 6 support website. I've also submitted a support ticket, but, it doesn't sound like this is a widespread issue. I can play audio from the MacBook if I use the headphone port and plug into the the aux in (17/18) port on the M20d.
Just wondering if anyone else is having similar problems?
I use the audio streaming capabilities to mainly capture rehearsals, so, it's not a show stopper as the M20d has so many other technological/beneficial qualities and I can still record to an SD card, but, it is disappointing. Looks like I may have some extra time this weekend to look into this problem more thoroughly. If I find a solution, I'll post that as well.