I am willing to bet it is the upper midrange you are not happy with as the tweeters are all the same in Axiom's line up so the trebble range should be good. The lack of midrange and lower top end would be due to the M50's design. I think you would be in for a treat to get some M60's, or if you can afford to go further, the M80s.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70