Here's a guy I follow all the time, though he doesn't post too much any more. The groundwork is already done.
http://zaphaudio.com/Anyway, here's what cone ring and breakup look like on a waterfall RTA

And it manifests differently on a harmonic distortion plot

See, this woofer works OK (not great) up to around 2k or so, and then there's this NASTY breakup/ring that occurs around 6k. And again, way up high out of the 20k bandwidth of most ears. We would actually "feel" that speaker being grating. This is a cheesy MCM-55 woofer btw.
freq response for reference

Here's a better one, a GR Research M165x
Waterfall

HD

and freq response

This woofer would probably cross much higher, with a lower "le" and a cone that is not susceptible to breakup, I'd wager 4k would be the sweet spot, on a steep slope.
By this same token, tweeters must be crossed properly too. Not just excursion issues, but also distortion expectations at lower frequencies.
Another fun variable of multiple points of output for a given frequency is the nature of reflections/refractions. A woofer in the door deals with those in a much different way that a tweeter on the A-pillar. All adds to the fun.
LOL Now you guys see why I "push" the processor I do. It gets maddening sometimes, thinking about all the variables. Fun for the tweaker though.