playback engine - optical format spdif problem
 

Its been suggested to me that setting the optical format (via playback engine) to spdf (rather than adat) reduces the load on the pci buss.
I've tested this & found it to be true. However, I lose input into my aux return (setup as an FX return from an MPX 500 digital out).
Ive double checked I/O setup, it looks OK, so why no input into PT via spdf?