Ok, better then most but R53 will not last much longer and might even be way out of spec by now. It's a 100R so any 2W resistor would work there as 1/2W is clearly not enough.
The preamp output is taken from the headphone jack but the ground is shared with the input sockets so you might be dealing with a ground loop. They are more likely in the P2P wired amps but certainly possible in these amps since Mikhail never could solder worth shit. If you a 1/4" to RCA cable then you test if it is the ground that is causing the hum.
You also have no input selector so the inputs are all connected together. You need a 3P4T switch for that which is dirt cheap and simple enough to wire up. I would also cut the loudness taps on the volume control if there is any danger they are shorting to ground.