Got the Cut control squeal issue fixed. Turned out to be an error in the wiring diagram which switched the two cathodyne PI outputs to the two 6V6 grid pins. That turned the negative feedback into positive feedback and caused the oscillation when turned up sufficiently high. Swapped those two wires and it's all good now!

Here's a new photo of the chassis with the squeal fix made. I also extended, rerouted, and did a solder splice on the orange single-ended OT primary and got rid of that wire nut, as well as installing a new set of tubes from what I had on hand. Under the preamp tube shields are a next-to-new EICO by Mullard 12AX7 in V1, a NOS BEL 12AX7 in V2, then a pair of current production Tung-Sol 6V6GT's, and finally a NOS GE 5Y3 recto. The amp sounds fabulous. Clearing up the positive feedback wiring error dropped the amp's gain down to a more reasonable level and it's got significantly more clean headroom than at first, while still having plenty of juicy filth on tap at the higher volume settings.