10v supply is "good"? So zero ripple on each? How about the 15v supplies, how much ripple on each?
You have a bunch of JFETs that serve channel switching related mute and enable functions. My suspicions point to them. Note the B and A volume controls each have one. That might explain why the other channel control still has effect.
Also, my general practice is to go down the row of ICs. I look for any DC offset at each output pin.