I hate to use the catch all, 'grounding issue' but check to make sure the stuff which should be isolated actually is, and that your signal grounds allmeet at the negative side of the volume pot. cathode return, negatives from the RCA jacks( input and output ). then take one connection to the capacitor's ground/negative lug.

And thenagain, there is always the silly unknown. I did somethig which appeared to be comletely unrelated to the linestage circuit and the hum I thought was ac filaments in my amp got remarkably quieter...
regards,
Dogulas