I am beginning to suspect the short drop several weeks back that broke the NP battery tabs off may have broken something else internally. At last night's rehearsal recording I had 4011's on ch 1-2 and 4006's on 5-6 and the right 4006 channel (F6 input 6) recorded about 6 dB lower than the left even though the input levels onscreen at the time showed no imbalance. I thoroughly tested everything today - mics, cables, recorder settings, etc. There is absolutely no problem I can find with anything.
This problematic M/S recording was from the concert I recorded right after the drop occurred. At the time, I (and many others) were suggesting the fig8 pattern on one of the 414s could be wonky, but everything tested perfect. That pair of mics was recording on F6 channels 3-4, with my 4006 pair on 5-6. Different cables, too. So I can't even track it down to one bad channel.
So I now have two different concerts post-drop where 4+ channels were recording where I have had channel imbalance issues in my recorded tracks. The cables and mics for the problem tracks were different, and everything (including the F6) tested perfectly after the fact both times.
One thing occurs to me: All of my testing trying to track down the problem after both incidents had me recording only two channels, rather than 4 or 6. I'm going to format my SD card and see if that might be a possible failure point. There are never any dropouts or glitches, though.