Crossover varies with the tops, subs, and the room. There are way too many variables to recommend a one-size-fits-most PA crossover point. Once you've established where the generally optimal point lies, the speakers, crossover included, have to be tuned to the room by tweaking. Most crossovers are either set way too high or low: too high is easy because most subs will happily reproduce low mids, but the lows suffer (though most folks never notice), and too low of a cutoff frequency is bit more complicated; the subs will generally be swallowed because in general with consumer MI stuff, you need four times the boxes and power you have for real true lows. A lot of times in troublesome rooms, a parametric EQ is employed WITH the house graphic EQ: the graphic can be set to the raw speaker response and the parametric compensates for the room- BUT just like gain staging, doing all this stuff properly is an art unto itself. I've never had to use one for that reason, although I have had to dial in massive cuts on digital EQ's that have both using both at once for really bad upper midrange spikes (common in rooms, mics, and speakers). The mix itself should be tailored to the acoustics of the room and many, many "producer" soundguys seem to have no clue that what comes out of the PA speakers should augment rather than overpower stage volume that is already there.
