Hi doormat,
Well, that's too bizarre! Actually, are you sure the M2 is in phase with the M3s? Listening to out-of-phase audio can sometimes induce a kind of nausea. I can't use noise-cancelling headphones because the noise-cancellation circuitry, which operates out of phase, gives me a queasy feeling. If I switch off the noise-cancellation, the headphones are fine. (I've experienced the same phenomenon with dipole surround speakers.)
OK, that's one theory.
Theory #2: Has your brother had vision problems? Does he need glasses and not realize it? Or if he does wear glasses, perhaps his eyes have changed and he needs a new prescription. Eye strain can certainly induce headaches and similar anomalies.
Regards,
