Riffman,
This has nothing to do with the tweeter or driver materials in the M60s, M22ti's, etc. As JohnK and others have pointed out, it is room- and source-related.
The M60s, M80s, M22s etc. are very "linear" speakers--a smooth frequency response. If a microphone that has an upper-mid resonance is used for the pop/rock recording (very common; some performers request mikes with mid presence peaks), then the Axioms will reveal that.
An upper-midrange resonance can exist in any speaker, regardless of the tweeter or cone materials. I have some older speakers (I won't mention the brands) with silk-dome and cloth-dome tweeters that have a spiky resonance that makes them unlistenable (for me) with brass instruments, violins, or pop recordings that have any tendency to sibilance.
Regards,
