Try to move your preamp by a couple santimeters different directions. You're probably catching some AC or your left channel stays closer to the AC than the right one.
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In vast majority of cases poweramps MUST stay away from all other small signal components.
How long you're listening to AE3? Did you try swapping tubes?
You can try first to get some cheap longer RadioShack gold interconnects(BTW they're great!) for the sake of getting your preamp away from poweramp. Also make sure that poweramp is not standing too close to the AC-outlet. A strong and shielded powercord on your poweramp will be a great plus.