Make sure you have the inputs for your interface set to SPDIF. If not, it won't recognize the signal. My M-Audio interface has a user interface on the computer that lets you select inputs and outputs and set their levels. Not sure if this is the case with the Mbox. Also, if you are going into a DAW you also have to make sure that the input for the channel you have selected is set to the proper input of your interface. For instance, for my M-Audio 1814, ProTools recognizes my SPDIF inputs as stereo input channels 9/10. So if I use SPDIF, I have to make sure to set this in the proper channel or ProTools won't recognize the inputs. I started off using digital outs, but like the flexibility of the 8 outs. For the hum, be sure to "invest" in high quality cables. Sometimes we spend $2000 for a digital sampler (MPC2500), with these awesome converters in it and then run the sound out of low quality unshielded cables and kill the beauty of the analog sound from the MPC. Don't get me wrong, digital is beautiful too, but a good set of Gold balanced Monster cables going out of your 8 outs will make tears come out of your eyes, especially when you hear an 808. Using cheap cables, it sounds like sh!t with lots of hums and cracks. Just a thought. It may just be your cables.