With the amp on and humming, use a chopstick (or similar) to move the leads around a bit and see if you can reduce the hum. Lift the 5.3v twisted pair away from the components and see if that helps.

Confirm that you only grounded one end of the shielded cable(s). Like the one going from the input jack tip lug to pin 2 on V1.

Beelzebum’s simple signal tracer sounds good, be sure to heed his warning about insulating the cord tip. Use one hand to trace the signal and keep you other hand away from the amp chassis.

I did my 2Stroke heater circuit like this and there is no hum at all from it. http://www.tubeampnetwork.com/tan-community/74-robin/photos/photo?albumid=4#photoid=97