Ive just picked up one of these Zoom U44s in gear4music sale (£80). I can get it to work with AX8 SPDIF in standalone mode (so I can use it as a headphone amp), but cant get SPDIF sync when its working as either a PC or ipad soundcard. I suspect its a bit rate issue (48k AX8 vs 44k PC and ipad default). What are you setting on your PC to get the AX8 SPDIF and PC both working?
You might have to turn the ax8 off and back on a few times - can be temperamental.
I set reaper to 48k to match the ax8.