tobz67,
Just to get this one out of the way, speaker break-in is a myth.
If you want to read more about this psychoacoustic phenomenon, try out a general search on these forums. You will find plenty that has already been discussed.
That aside, i suggest you read through this
relatively recent thread on this very subject of first time speaker experiences. What you read in reviews and from other users is not always what will happen or will become your experience as well.
Keep in mind, you are very used to hearing a certain sound from your old speakers that ANYTHING new will sound different and not necessarily "blow you away" better....simply different.
That being said, the M60s are apparently more bright (less laid back) than the M50s. If you desire more bass, then the M80s might be one way to go, otherwise add a sub to your M50s/M60s for lower end oomph.
Personally i'm not a fan of big bassy sounds in music (unless its required as with a pipe organ) so the tight bass of the Axiom M60s I find very pleasant.
