It's possible your interconnect cables are running too close to the power transformer on your tube preamp.
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A slight hum coming from a tubed pre-amp could be from poor contact between the tube pins and the socket, or noisy tubes. Try removing and replacing tubes, tensioning the contacts in the tube sockets if you can, or having a spare pair and change them out and see if there is a difference.
And perhaps Hifigeek is correct and it is nothing much more than cable and IC dressing. I have an amp which is very sensitive to PC placement. Try to keep PC's and IC's not only away from transformers but running parallel with each other (crossed should be OK).