Hi Andy,
You've tried exactly what I would suggest - swap the cables and see if the fault moves to the other speaker. As it stays in the same place, this does sound like a speaker problem. You can remove the amps from the base of the Pentas and check the outputs with conventional passive speakers, as they just have standard red and black output terminals, plus a DIN plug for the display. This will tell you if you have a problem with the amp, or if the wiring inside the speaker needs attention.
By the way, is that you sailing an Enterprise in your avatar? I learned to sail in one of those, they're great boats!
Also, thanks for your kind words!
Steve.
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