This depends on the woofer. Some woofers _have_ to be crossed at 2KHz. A lot of woofers play into very high frequencies. For example, all Hybrid Audio woofers can play into 5KHz and higher. Often active users cross them at 6KHz or so. This is entirely driver dependent. There is enough test data to see the usable frequency range of many popular drivers (e.g. DIYMA klipper tests, zaphaudio.com, etc). My personal preference is to stick with woofers that can be crossed high, leaving tweeters to just play the treble. This should be better for imaging if the tweeter is far apart from the woofer, and most tweeter distort below 4KHz anyways. I'd experiment with 2-way speakers that can be crossed at around 2Khz if I had a high performance (large format?) tweeter and it could be placed near the midbass..You guys are giving the dumbest crossover frequencies I have ever heard. The correct answer is somewhere around 2.0kHz to 3.0kHz. No more, no less. Higher and you get some nasty breakup from most 6.5"s plus very crappy off-axis response plus you want your drivers to ideally be flat 2 octaves (four times the frequency) past your crossover, but at least one octave. Also keep sensitivities and locations of the drivers (speakers, not the driver of the car, haha) in mind.
