I don't know if I would describe it as added warmth, rather a more lush midrange. It brings out or emphasizes the harmonics in the recording as tubes often do.

I believe the M60s, as good as they are, aren't the smoothest speakers out there. A lot of people on this forum have stated that sibilance is in the recording. While I agree with that, I have heard speakers that sound just as detailed as the Axiom's with a smoother, more natural upper midrange through top end. Granted these speakers cost 3-4 times as much!

I listen to a variety of music and consistenty the metal/alternative rock doesn't sound great on the M60s. My shop system does well with this type of music, but can't touch the M60s soundstage, resolution, etc. with the right recording.

If the genre of music you most often play doesn't agree with the speaker's frequency response it may be time to look at another speaker, posibly the 50s?