Hi Mark,
I spotted my name in your query so, although I'm retired from Axiom, I thought I'd mention a couple of factors.
I don't believe I ever did listening tests of the M60 v4. If memory serves, I think the last version I heard was the v3.
In passing, I'd note that whenever Ian upgrades an Axiom model with a new tweeter, he will always tweak the crossover a bit to adjust it to the characteristics of the new tweeter and achieve a smooth neutral frequency response. In past listening tests, the main improvements I heard in upgraded M80s and M22s were improved spatial presentation, a more convincing "3D" image with greater width and depth, and I'd expect that to be the case in the v4 upgrade. I'd describe past improvements as "subtle but audible" in the various upgrades I heard.
I'm a bit troubled by your comments on your M60s' "sometimes slightly harsh highs/female vocals". Going way back to the original M60s before the various upgrades, that was certainly one of my constant criticisms of the M60s. I heard it on vocals and horns, a kind of edginess that bugged me. But that seemed to have disappeared in the last blind tests I did; I recall writing down "this is a wonderful speaker!" and I'd certainly never said that about the M60s in previous tests.
Perhaps Ian might be able to find that listening test I did of the M60s a few years ago and report which version I was listening to -- it was likely the v3.
Generally speaking, the newer tweeters have better power handling at very loud playback levels, and hence sound smoother and less stressed than previous tweeters.
You'll have to make that decision about the v4 upgrade and whether it's "worth it."
Regards,
Alan
