Martin Logan SL3 repair

"...So is washing panels an ML sanctioned thing?..."

Probably not but what do you have to loose? Many have done this successfully.  


MartinLogan | Help Articles : Features and General Questions for Electrostatic Speakers

Martin Logan Quest Stator Panel Shower Washing (martinloganowners.com)

If stator is the problem, Martin Logan will build you brand new stator with updated spec. But it will cost ya. 

As a former SL3 owner until last July I would  say if these are the original panels they are probably past done their useful service.  I had mine from 1998 ( repanel 2010)  to present.   They still sound great but decided to get 11A's when ML had the sale last July.  
Repaneling is costly.  I  am awaiting CLS panels($ 2995)  which took 5 months. I have the same issue , washed them, no difference. Just a dead panel  that is 20 yrs old. 
Suggest looking at the new ML's  as the new panels have improved, better bass integration,etc.   Good luck