Bruno, I also was very happy that the supposed volume problem was solved, and I was terrified by the thought that if we tried to analyze it in more detail, it would re-appear.

Nevertheless, in what you describe, the crossover setting with the M80s set "Large" would be of no significance unless a "+" type of setting had been selected(no indication that this was the case). Otherwise, the EP500 would play no main channel bass at all, regardless of the crossover setting. I'll take the risk of speculating that an unsatisfactory bass level(because the sub was playing no main channel bass)was masquerading as an inadequate overall volume level, which was corrected when the EP500 began to play the main channel low bass.
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Enjoy the music, not the equipment.