Hello Jon

Not sure if my experience is relevant but I have a pair of M100’s which had the electronics refurbished but when I reassembled them one speaker had no treble? It turned out to be a pin on one of the two connectors that connected the amplifier output to the speaker enclosure, the pin had pushed back in to the connector slightly causing the fault, Chris at Meridian Repair Shop remixed the pin and it was fine. I wonder if your issue could be similar?

Russ