A fellow wrote the following comments about active/passive crossover
design and speaker design in general. Hoping that Dick Pierce and
others skilled in the art will offer up their comments and
perspectives.
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If you have a calibrated microphone, then just load up Speaker workshop
or some other MLS measurement software into your computer and you're
golden.

I wouldn't try designing active crossovers by ear or even with
something like a Radio Shack meter or scope, but once you get a good
handle on the sound of the system and your drivers (by listening a lot)
it's a good idea to adjust that way. I ended up with something a little
different from the "ideal flat frequency response" when I tweaked my
crossover by ear.
A lot of speakers rely on their passive crossovers to equalize out
anomalies in the drivers and crossovers, but you don't always need to
do anything besides the driver rolloffs and maybe a shelving filter for
the baffle step. I have enjoyed the results of speakers made without
the crossover "equalizing" pass band responses, which is usually
unnecessary with hi-fi drivers and good selection of crossover points.
I just got to listen to a speaker which was made with an Alesis plate
amplifier - completely textbook, 8th order active filters at 1.8 KHz -
and some very nice Parts Express drivers. Except for a tweeter-woofer
level mismatch which I think the owner just corrected, they sounded
really good. It is possible to avoid an involved design process, with
the right components.