eduardo, I think the M40s sound "laid back" because that's the way Axiom describes them
here
"The M40 adds deep bass extension to a classic two-way design. This slender, elegant tower effortlessly produces smooth, laid-back music for a pleasant listening experience."
The M50s description says "You'll be thrilled with the clean bass produced by the M50's twin aluminum woofers! Its titanium tweeter delivers silky and detailed highs with a smooth, pleasant midrange."
M60s - "If you'd like the startling definition and realism of the M80 tower in a more compact, 8-ohm version, check out the M60s. A three-way system with triple vortex porting, a dedicated 5 1/4-inch midrange driver, titanium tweeter, and dual aluminum woofers, the M60 effortlessly reproduces the full musical spectrum."
Each has been designed to have a different sound.