The so-called "shrillness" that you experience isn't coming from the mid-range. You can try felt but also try some silk in front of the tweeter.

BTW, where did you find out that the M3s have a mechanical cross-over? I've often wondered how the Axiom cross-overs are designed. I wouldn't be surprised if Axiom makes use of mechanical cross-overs in all of their lines. Hence the relatively high efficiency and power handling.