I'm stumped with this.
The signal comes in. Is present at the base of Q80 & Q82, and then drops to a very low signal at the collectors of Q80 & Q82. The very low signal is at the base of Q75,76 and Q77, 78 and gets amplified back up at the collectors of Q75,76,77,78.
On the other (working) channel, the signal at Q80,82's collectors is about the same as their bases, so by the time it gets to the Q75,76,77,78's collectors it's many times larger.
I just can't figure out why this is happening on both Q80 and Q82. I swapped those transistors with the good channel just in case, but it's exactly the same.
All transistors are getting their power from the +60 & -60 rails.