With regard to the blind listen test, I can't give you facts because I've never compared the same speaker with and without HP but I can give you a reasoned opinion.
Axiom didn't introduce a three-way bookshelf until the HP driver was ready. We can speculate why. Maybe all the products in the Millenia line were doing just dandy and there was no market share to capture with a 3-way. Maybe Axiom felt a 3-way would be too big, confusing prospects who couldn't identify with a speaker that was too big for a bookshelf and too small for a floor-stander. That doesn't sound like Ian though. More likely, the standard woofer just didn't have the 'nads to keep up with the mid and tweeter. This is why the M60 has dual standard woofers. Once the puny, standard woofer problem was solved with the HP, the M5 and QS10 were born.
Now we can ask which is better? The M60 or M5? I don't know. I've never listened to an M60v4. What I do know is Ian prefers the M60 to the M5 because the M60 is more sensitive and hence easier to drive. Based on the published curves, the M60 is more linear too. Does it disappear as well as the M5? I don't know but the M5 is quite inert and small enough not to get in its own way, resulting in acoustic holography. And the M5 gives the warmth the M2 is missing but you have to be careful with room and placement because the warmth can become bloat. Note the M5 has a similar bump as the M3 down low. It's like Axiom decided to blend the best of the M2 and M3.
So the last paragraph tells us that dual standard woofers can do what one HP driver does with higher sensitivity to boot. But that requires a larger cabinet which comes with its own set of acoustic resonance problems. You gotta know which peaks and dips matter and which don't. If you don't know how to deal with that, you end up with a $50,000 3-way that is made of lead and lined inside and outside with no-rez.
As for the mid having a wider bandwidth, it doesn't matter until you start to ask for more SPL. Then you just add another mid-woofer like the M80, M100 and QS10 have.
You don't need a sub with the M5 for music but it sounds so much more emotionally satisfying with a sub!
