I decided that lash-up too whisky. The digital seems quite well separated
from the analogue on the PA board and 2 lines seem to come from off board to
p2 and p6 of an M5218 dual op-amp SIP via electros from CN1 p6 and p8.
Injecting 0.1V in those points carried through to both the speakers. That
CN1 header traces back to the display board with vol and balance pots on it
so a job for tomorrow