I'm pretty sure it's all the woofers are different {Bigger}not just Axiom white Bryston in Black. The amount of excursion to make the extra SPL is likely extreme and somewhat hard on the drivers so they would need to be really robust for such extreme efficiency. Same as the cabinets need more body to absorb such excursions.
I myself have gone from M3 to the M22 now to M80. Have found each model to meet and exceed my needs without any deficiencies.