Galvanic isolation is a step in the right direction - however its only providing perffect isolation at DC. Check the interwinding capacitance of your S/PDIF transformer - the ones with the best isolation sport a figure <1pF. If yours is higher you still might gain improvement in fitting a CM choke to your cable (or clamping on a ferrite). Also check to see if internally there's a dedicated ground wire going to the digital input termination (75ohms) from the mains trafo secondary. A lot of DACs just use their internal groundplane to carry the CM noise current, impacting the sensitive analog circuits using that groundplane as their 0V reference.
Years ago my friend who was heavy into audio used to laugh at some of the cable specs and reviews in the audio magazines. One vendor actually said you should connect their cables between the amp and speakers and run them for some number of hours, then disconnect them and swap them end-for-end to continue. Something to do with "conditioning" the cables. He was sorely tempted to start his own cable company with some high-quality mike cable and silver-solder it to connectors.
As for digital, I hate digital TV. In the old days you could still see through a little snow and hear through a little static, but now it's just large chunks of picture and audio dropping out. Verizon just had to replace the terminal unit at my house because of noise or signal loss on the upper channels, making them unwatchable (and unrecordable for time shifting). I need my "My Favorite Martian" fix!