[Techman]

And I'll add to the above in this separate post - I think it would be wise not to individually 'tack' separate capacitors across just one of these that are in series at a time for testing (now I've studied the circuit). You should do the test (if you're going to do it) on the others in the circuit first and think about the reservoir last, if you still haven't identified the hum, which I'm definitely now thinking is going to be caused by one or both of the ones you've identified in your picture.