Richard, if you mean that you're comparing the M2s alone to the HPM-900s alone, then the bass extension and loudness capability of the HPM-900 with its 12" woofer would be a clear area of difference. The bass extension(at least 20Hz lower)of the M22 would in part change this.
I'd also note that for my 2-channel music listening(well over 90% of my listening)I never limit myself to just front speakers if a full setup with sub and surrounds is available. Using a mode such as DPLII to extract the surround ambience that was mixed into the front channels(there was no place else to put it)and direct it to the surrounds where it belongs increases the realism and enjoyment of home music listening significantly.
