different crossovers, which to use?

ok, thought I would bring this up since it seems to be a recurring question that has been popping up and I'm sure many are wondering the correct answer (including myself). 

Nowadays, most head units have their own electronic crossover's, amps have their own crossovers, and speakers even have their own (passive or active). What is the correct procedure to use to get optimal sound out of all these crossovers? Use the headunit? Use the amp, use both?? I think it would be helpful for those in the know to post some theory behind this. If this has been beaten to death in the past, then please excuse the re-posting. Cheers.

My 2 cents:  NEVER cascade crossovers.  You will end up with unpredictable effects.  If a loudspeaker is supplied with a passive crossover, always use it unless you are bi-ampling (or tri-amping) and fully understand the performance characteristics of all drivers involved.  If faced with the choice of using an active crossover built into an amp or one built into the head unit, I usually default to the HU if possible as the electronics are usually better and the installer can adjust all settings from one place.  If possible, use neither but a dedicated outboard crossover instead.  The crossover settings and slopes between any two speakers in a system should be at the same points, meaning if the sub low pass is set to 80Hz the mid-woofer high-pass shuld be set to 80Hz, cross between mid-woofer and tweeter shoud be the same, etc.  You want there to be no "holes" in the sound spectrum.  That being said, it is also normal to adjust crossovers away from these starting settings when fine-tuning a system's response, but only if you have the proper equipment (like an RTA) and know what you are doing.

Cheers