I agree with the crossover problem. There is a power resistor in series with the tweeter. If it is failing, the resistance rises with temperature and the signal voltage is converted to heat instead of music.It wouldn’t cause this explanation, just muted high frequencies, he would still have all the bass and midrange.
" will exhibit significantly lower volume (down to almost no volume)"
Try another amp first, as these Extremas are put together like a Swiss watch, it would be a shame to break/damage all the seals to find out they’re fine.
Cheers George