It is hard to say withought having listened to both as to how subtle the difference is. Subtle is a very relative term.
The M50 is the only speaker I did not audition when at Axiom, so I can't even tell you what difference I perceived. Perhaps most of my aversion to the M50 comes from listening to the Paradigm Monitor 7 vs anything in the Studio line. There was a BIG difference there and I did not like the 'laid back' (read muffled) sound of the Monitor 7.
I personally like to hear the string plucks and the details of a bass string vibration. That addition of texture is the difference between sounds good and sound real. It is what makes the speaker start to sound transparent: disappear so that you hear music from a band, not speakers. That starts to come through in the M60 and comes through in spades on the M80. I do not know how much of it comes through on the M50.
That is one of the differences I noticed between the M60 and M22. The upper end on the M22 was more transparent. I would imagine the difference would be more pronounced between the M22 and M50.
The down side of all this detail is that poor recordings will sound noticably poor. I notice it with two of my favourite recordings: Yes, Close To the Edge and Yes, Fragile.
I do not know if my ramblings are confusing or helping at this point so I'll stop.