Hmmm, this is puzzling. Do you have a sound level meter, or an app for your smart phone (lots of free ones available)) so you can measure what sort of playback levels you are using? The M22s will normally play plenty loud in average-size rooms like yours without distortion driven by ample power amplifiers (75 to 100 watts per channel).
You may be listening at extremely loud levels that are stressing the M22s and causing the distortion you are hearing. (Some former colleagues of mine used to listen at levels I'd call "deafening", peaks in excess of 100 dB SPL, which would cause me to leave the room.)
Since you are driving the M22s full-range, then they could be distorting at such levels, in which case, larger floorstanding speakers like the M60s or M80s would play distortion-free so long as you have plenty of amplifier power to feed them.
As a rough guide to subjective loudness levels, 80 to 85 dB is "quite loud"; 90 to 95 dB is "very loud", and above 95 dB is "extremely loud". Most people are comfortable with playback levels of 80 to 85 dB with rock or classical music, with occasional peaks to 90 dB or so.
Changing the crossover on the subwoofer won't help matters for the M22s since you are driving them full-range from your Nad's amplifier output.
Alan
