Removing the preamp stops the hum. All of your circuit design issues regarding grounding and loops are not to be ignored.
However as an owner of a couple of tube pres including a Cary Low end brand. I can tell you that a bad tube can cause a number of different noise issues. I always took the stock 6SN7s out and rolled in a wide variety of old stock. Man oh man I couldn't believe how many fine testing for transconductance when put into a preamp how noisy and bad tubes reared their ugly character.
Thus my question Have you experimented with the tubes? How many hours do you think you have on your preamps tubes?
Obviously you know what I am going to suggest you do. Roll some different well tested tubes into the pre. I know the other sugestions seem more plausible at first glance, but so far grounded has not changed you problem
However as an owner of a couple of tube pres including a Cary Low end brand. I can tell you that a bad tube can cause a number of different noise issues. I always took the stock 6SN7s out and rolled in a wide variety of old stock. Man oh man I couldn't believe how many fine testing for transconductance when put into a preamp how noisy and bad tubes reared their ugly character.
Thus my question Have you experimented with the tubes? How many hours do you think you have on your preamps tubes?
Obviously you know what I am going to suggest you do. Roll some different well tested tubes into the pre. I know the other sugestions seem more plausible at first glance, but so far grounded has not changed you problem