Different crossover frequencies such as those you've seen are needed because you want to split the signal between two speaker units at a frequency both are happy with, and this point depends on the combination of speakers being used.
This can be quite an art with passive loudspeakers because as well as getting the crossover frequency correct you may also have to compensate for the differing sensitivities of the speakers so that the overall sound is not unbalanced. Building an accurate high-power crossover is quite an exercise due to the physical size and weight of the components you need as undersized bits may well have the necessary response characteristics but can impact negatively on speaker performance at high volumes and at worst could fry and fail.
Also there are a number of different ways you can design a crossover, which will give steeper or shallower curves at the crossover frequency, and also may result in the signal to the two drive units being in-phase or out-of-phase at the crossover frequency. When done badly this can make things sound somewhat "wooly" at the crossover frequency. This also explains why some passive speaker systems appear to have the treble speaker wired up the wrong way around this is done to avoid phasing issues at the crossover frequency.
With active speakers life is actually bit simpler. It is much easier to build sophisticated crossovers at the input end of the amplifier than it is to do the same when built into a speaker. You have lots of control over the internal gain of the amplifiers to achieve a smooth response. More advanced active speakers will probably have some sound shaping built in to smooth things out even more.
Given that quality active speakers are now becoming quite affordable, and also can be surprisingly light thanks to advances in amplifier design, I think I'll be going active when my current setup expires.
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John Allcock
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