I beleive it would not help using volume offset as he gets extreme distorsion, using normal volume settings per channel, when cranking up the volume. If it's possible to have louder audio in FM mode without distorsion then it's really strange. If there was a subtone in the audio it would overload the amplifier and speaker and result in that problem but airband doesn't use subtones.
But just as a test to get rid of any unaudible tones set CTCSS to Search for that search range or channels as that will engage a 250Hz high pass filter. I assume firmware upgrades have been done and the problem persists so no point in doing a reload. And when monitoring at home and compare FM with AM and what highest volume could be used, AM doesn't sound as loud without going into distorsion?
/Ubbe