That's a clean and neat PCB!
Heck, just shortening those wires (just to-from the input, volume, and tone) should kill the squeal. Route the other wires away from the preamp and use a cable tie to keep them bundled away neatly.
Don't worry about the heater wires' angle. As long as they're twisted you'll be fine. They do not carry signal, but they do run AC voltage (AC will give you low frequency 60 cycle hum).
Since the amp is cathode biased, if hum continues to be an issue, there's one or two more tricks we can still use...
3 Gibsons, 5 Teles, assorted other guitars, about a dozen amps, about two dozen pedals, a Smith & Wesson SW40VE, & a .40 SIG Sauer P226R = too many toys, no money, carpal tunnel, and a serious hearing problem.