A lot has changed, and much of it is unknown to us. Axiom has experimented a ton to extract as much fidelity as possible from the Millenia architecture while preserving capital investment. This is my educated opinion as an outsider looking in.
I'm sure your M60s can handle 250W "average" with 1000W peaks. The fundamental architecture of the drivers supports these numbers. The question is "to what levels of performance?" There is a big chasm between function and performance. And it is this chasm that Axiom has narrowed.
The free-standing cabinets were beefed up and I think any audible remaining resonances were treated within the realm of practicality given passive filters. With the actives, some of these resonances were further addressed. With the Brystons, Axiom went even further and the price reflects that. I would expect the Model T acoustically disappears better for example than an M100 and it is also more visceral.
The mid and tweeter were re-designed partly so they can handle more power. There is a big difference audibly between v2 and v4 in this regard. That doesn't mean one can't like v2 mids. But v4 mids are more emotionally impactful.
The only reason for the HP driver is to play louder with less distortion - particularly at the lower end of the woofer's range. That doesn't mean the standard driver is crap. Axiom has never quantified how the standard vs. HP drivers compare. But I am sure if a prospect was to describe their listening room, system and habits, Axiom can make a reliable recommendation.
Ian has said he prefers the M60 with the standard drivers more than the M5. He's also said an M2 with sub is like an M60. I think with the current sub technology, M2 with sub is better than just an M60.
