"I tried the Transit on two separate Apple systems myself, and had several friends come to the same conclusion on different systems.
....the music sounded like it was castrated."
I heard back from M-Audio and they have verified that "in order to match levels of the MAC to PC, the MAC digital out must be set to +8."
I recently tried the comparison of MAC to PC using my Off-Ramp at CES, and initially it really sucked, but after turning off all of the DSP functions of the MAC, it got a lot better. The dynamics and loudness was lower though. I think the +8 setting explains this. M-Audio said that the driver fro the Transit for MAC is actually superior to the PC version.