Reseat all your connections and check your cables (especially the one from pre to the HK) for intermittent shield.
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I had a simalar issue with my tube pre and if I touched the chassis you could even hear my finger touching it..I thought it was a Microphonic tube..It turned out that it was a bad connection ( internally ) with the IEC connection..Anything on the preamp ( including tubes ) was hyper sensative to touch and actually vibrated and echoed big time..Once the issue was found and the IEC repired, the issue was solved..Its very possibly you have a bad ( or faulty ) connection creating the problem....
What is the intermittent shield. Is IEC internal electrical connection? and how can I remedy this? Cables seem fine as well as connections. Even swapped another power amp to see if this was the problem and this too was humming loudly. I also switched out tubes on the pre with no change.Opened the pre and saw no visible signs of any problem. The headphone amp works so Im thinking the problem is with the tube pre amp.The only thing I see is when I jiggle interconnects from preout to amp in I get humming coming in and out.