I've found 80Hz works best. With my 100s, I used to switch between 80 for movies and 60 for music. With the M5s, M50s, M2s and M3s, I use 80. I also find it difficult to align speakers with the rest of the system when the speakers are playing wide open. Group delay comes into play and the bass does not sound as "fast" as it can be.
Some say "I got these great speakers that go down to 30Hz and I paid for that and want to use them down that low". I say if that works in your room, go for it. Otherwise, rest assured your speakers will go down that low, even with the 80Hz cross-over, and the entire system consisting of room, speakers, electronics and ears-brain will work better. The speakers just won't be as loud down there and that's OK.
I've found the actives play wide open much better than any other Axiom. Nonetheless, at an 80Hz cross-over they sound better particularly with multiple subs. They play much better because they are more linear. There's only so much a designer can do with passive speakers and I'm pretty sure Axiom optimized the v4 as best as anyone possibly could within the current architecture. I realized this immediately when I first got my M5v4.
