You've already got a pretty nice HT system; I guess the main benefit you would get from upgrading to Axioms is that same "sweet music system" performance you heard with the B&Ws (M60s are often compared to B&W 703s).
From my own limited experience timbre matching between surrounds and mains/center seems to be vitally important for music but less important for HT. I ran quite happily with M60 / VP100 and some crappy old tower speakers as surrounds and never noticed a timbre mismatch on HT, but playing music in 5.1 sucked. When I switched to matching surrounds (initially M2s, now QS8s) all of a sudden 5.1 music started to sound wonderful...
I have not heard QS8s vs. bipole/dipole surrounds (just vs. direct speakers) -- I know the difference is dramatic going from direct to quadpolar, but I imagine the bi/dipolars would be somewhere in between.
The benefits of the QS speakers seem to be in three areas :
1. They do an even better job of "disappearing" and giving you the effect of more speakers in a much larger room
2. You don't get the side effects of bi/dipolar speakers, that "phaseshifty" effect in some positions when the drivers are wired out of phase (forget if that is bi or dipolar).
3. As a consequence to 2, they seem to be able to give you the best of both worlds -- good performance on music AND good performance on HT -- while di/bipolars (one of them
) tend to be discouraged for 5.1 music because of the "phasey" effects. I guess you can probably flip the switches when playing music but haven't had a chance to play with that...