Very simply put, xmax is the distance the cone moves linearly in one direction, roughly half the distance of the width of the voicecoil. Since, also very basically put, SPL's, particularly bass frequencies, are "moving air", the further the cone can move the more air in can move.
I'm still confused by the role of the speaker in "sustain", by my definition "sustain" is the amount of time it tales a note or chord to decay. As far as a
speaker is concerned, once the signal stops getting
to the driver, the sound stops. If you strike a chord on your guitar, and while it's "sustaining" you jerked the speaker cord out of the cabinet it would stop making sound in milliseconds. To quick for you to hear it, it would react like you hit an off switch.
Don't try this experiment with a
tube amp!
What you are doing and calling "bi-amping" isn't "bi-amping", I'm not sure what the term is, or if there even is one. Let's say "multi-amping" for lack of a better term. Bi-amping is using a seperate amp/driver dedicated to and optimized for a specific frequency range. Usually best used for sound reinforcement, as it is pretty complex to “get right” in a MI application, but it
can be done, and is very efficient in maximizing SPL output.
Here’s a personal example, this was my main gig rig for about a decade;
It’s bi-amped, with a passive xover going to the tweeter box. I run 650 watts to the 2X15, 400 watts to the 4X10 and the tweeters. It takes more juice to amplify low frequencies than high frequencies. The 2X15 is x’ed @ 125 h (+/- depending on the venue) and the tweeter box is x’ed @ 3K hz.
It’s loud,
really loud. However, to get it set up “sweet”, it takes a lot of time, almost as much time as tuning a big PA. I would generally pink it at about 100 dB for most venues, getting it flat using a 31 band eq (in the back of the rack) with the preamp eq set flat, and then adjust my tone with the preamp.
I still have it, don’t use it and frankly sometimes can’t believe I actually liked that “Seinfeld theme song” tone I used to get with it. It worked for the band I was in at the time, but I can’t imagine going back, I should probably sell it I suppose.
Anyway,
that’s bi-amping, what you are doing is far more common and a heck of a lot easier to deal with, but if you want
LOUD, true bi-amping is hard to beat.