Getting somewhere now. To save desoldering a shielding plate I'd been
probing the stereo channels. Problem is with 1 to 4 channels only, so
removed shield. +/-15V ok on those opamps. LED supplies are off the -15V
line, one zener per channel, giving nominal voltage of about -8.4V,
varying a bit on each channel.
Just pins 2 and 3 of each of the 4 off level double-opamp ICs show that
same negative voltage on each channel, the B opamp of each is normal.
Still a mystery what the problem in common is to cause that to each
channel, but zeroing in.