I'm pretty sure I can't make individual crossover settings on my Denon so I have never tried, but I think an M60 would be well worthy of the experiment. It probably becomes subjective to your tastes until you go so low that the low end starts to be underpowered. I know some have posted lower settings for their capable mains and some even make adjustments based on weather or not the content is movie or music.

With great power comes Awesome irresponsibility.