Rock, it's possible the more complex music you are listening to has a higher dynamic range. As a result, the M2s are being pushed into their non-linear operating range leading to greater harmonic and intermodulation distortion as John said. I've maintained the M2s with sub at 80Hz crossover are good to 12 feet MLP for the vast majority of listeners. I am curious what your sub crossover is and how far you're sitting.
BTW, have you confirmed that your sub is not distorting?
Large speakers are great as long as they don't overload the room and can acoustically disappear.
