Did the circuit board come as part of the kit? It's not helpful that it sits over the tubes, making wire runs overly long. You have used as much wire in that as I might use to build a twin!
Basically, I think that there are inherant flaws in the layout, I'd be tempted to just have the filter caps & power supply resistors on the board, mount any parallel filters on top of each other, trying to position them all as far down the PT end of the amp as possibe. Cut the board down & mount the preamp components direct to the tube sockets & on terminal strips, making sure everything mounts close to where it feeds.
I doubt that just moving a wire or 2 is going to make the necessary difference.
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