Onkyo av receiver woes

[dertoid]

Hiya all. Long time lurker, avid eevblog subscriber.
Here's my problem.
Onkyo RZ720 av receiver developed a hiss in L channel, which is volume independent, get a liiiitle lower when 'mute' is pressed, sometimes it starts making rather noticeable random sounds - like hissing gets louder static-like. All other channels are dead silent.
Headphones - pronounced hiss in L channel too.
So, i whipped out my multimeter and rigol and here's the result -
-0.35v dc on L channel. All other chans are below 0.01v.
Oscilloscope doesn't show anything significant (like ac 50/60hz leak etc) in AC mode, -0.35v steady-ish in DC cpl mode.
I have the service manual (can attach on req!) and narrowed it down to +29db PA on the L channel.
Yes, i did read info here about leaky mute transistors - but i can see in my device those are before PA stage? Or am i being horrifyingly noobish?
I do ok with digital circuits but all those 'class A' and 'offset bias' - and i'm lost.
Yes, i understand (looking at diagram) that all channels have 3 protections in place - a) overcurrent b) over DC (!) and c) over voltage (re +-69v getting thru to speakers from amp power rails). Looks like my -350mv are just on the verge of triggering protection b).
I also understand that my left speaker's coils are under undue stress (lovely magnat).

I would appreciate any tips, input etcetera.
Stay well and safe!