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.