Hey! Just saw this.
So I had this problem on a pair of brand new pair of ML ESL 9 Classic Masterpiece which I bought post covid lockdowns here in India. They were basically in storage in a coastal city for like 10 months maybe more. Unused.
After 3 months of use they developed the same 1.5db shift on one. Long story short
The problem can be
1. Bias boards
2. Panels.
First we did the same thing. Swapped amps preamp, physically swapping speakers to LR channels and sources, cables etc to disqualify all external factors.
Then they swapped bias boards amongst the speakers and to qualify if the problem follows the boards. However it didn't. The boards however on opening the speakers revealed a weird residue coming out of the transiators that had hardened. Either way the boards weren't the problem. The panels were. Humidity or storage under these conditions created dust or some damage to the panel causing the panels to overload and effect the bias boards causing them to possibly leak as well.
ML replaced both my bias board and panels as these were new speakers, had a huge tussle with them on email to give me new speakers. I would suggest you use the balance on your preamp and just temper them to match as anything else will and may create longevity issues on the set. Looked after well they will last for decades.
If you want to keep them find a service center or help to consider isolating the problem to either the boards or panels and eventually get replacements. Boards are not too expensive. Panels can be.
I eventually changed my amps to mcintosh mono blocks with auto formers after they sent me new replacement panels and boards came in, basically making the speakers good as new.
I was previously running emotiva amps which cannot handle the 2ohm swings these panels exhibit at the HF band.
Hope this gives you some kind of reference point to technically approach the problem.
Here is my original thread below
https://www.martinloganowners.com/t...ence-in-left-right-channel.18987/#post-198737