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Cables are original Meridian. I'll swap the speaker once more, but I'm pretty convinced it's not a cable problem.

Good luck, BTW.
I probably wasn't too clear.
A single faulty cable can cause this and it may not matter where it is in the system.
It just needs to be connected somewhere to cause the problem.

While unlikely, you've eliminated the likely causes.
Unfortunately, substituting all the cables to test this theory may pose a practical challenge.
Perhaps your local Meridian dealer has a box of cables to loan.