Oh dang!
I thought I had this faulty dynamics processor sussed when I replaced the LM7815 on it's split rail PSU. Turns out one of the channels is dead; it's showing a gain reduction with no signal present.
So tomorrow i'll hook it up to mu MotU interface, pop a hot 100hz signal through it and trace it to, hopefully, a faulty op-amp; with a digital multimeter!
Sheesh!