Checking for DC in the speaker terminals. Bank A left reads 8.3mV.
Bank A right reads 5.9mV. Bank B left reads 8.4mV. Bank B right reads
6.1mV.

Have you sat there watching the meter to see if the offset jumps up when the
relay opens? (I'm sure you have better things to do with your time!) Some
DMMs have a trending feature that keeps track of the highest voltage.

Arfa's suggestions are a good place to start.