Here I will try to compile a list of things I know, and from there we can try to narrow down the options for the noise source.
-The background hiss is independent of volume setting(with inputs grounded, there is extra noise otherwise)
-The balance control will cut out the hiss by turning it all one way or the other.
-Both the treble tone knob and the high filter switch has an effect on the hiss, though the tone knob not as much.
-Grounding J414 with a 0.1uF ceramic cap removes 90% of the hiss(what I would consider to be normal with vol control at min)
-Grounding J412, J410, J406 or J404 has no effect(again with the 0.1uF cap)
-Grounding J408 cuts the hiss down by about half(with the 0.1uF cap)
I am not sure what to make of this, how would grounding J408 have an effect on the hiss but not J412, which should be further down the line?
The other annoying noise besides the hiss is a 120Hz hum, which mostly goes away if I touch the chassis, could it be related to the 2.2M resistor R011 on the mains side of the transformer?