As someone else said, the crossovers depend on the drivers chosen and the overall design.
Back in the old days when speakers looked like monkey coffins, they made true 3-way speakers, meaning Woofer, Mid-Range, and Tweeter. In a system like this, for a variety of reasons, the crossovers were likely 800hz and 5khz. As a starting point, most 3-way designs assume roughly 3 octaves in the midrange. So starting at 800hz, then next octave up is 1600, and the one after that is 3200hz, and the last octave would be at 6400hz. That's pretty high well outside the range of most normal instruments. So, it is typically dropped down into the range of 4500hz to 6000hz, with 5000hz to 5500hz being common.
That's not typically how they design 3-way speakers today. Today, many Tower speakers are 2-way speakers with an additional Low-Bass driver. So, they would be Low-Bass, Mid-Bass, and Tweeter. Just to illustrate the Low-bass/Mid-bass might cross at 300hz, and the Mid-bass/Tweeter might cross in the range of 2500hz to 3500hz. These new style speakers could be true 3-way or they may be 2.5-way. Meaning the Low-Bass and Mid-Bass share a section of the very low bass range.
However, there are still more traditional 3-way design being made.
So, the intend and the design philosophy will determine the crossover, in combination with the specific drivers chosen.
With small diameter bass drivers you have to make a trade off. If you want deep bass, then you lose midrange. If you want midrange, then you give up low bass to get it. In a 2.5-way or 3-way Low/Mid-Bass design the Low-Bass driver can be specifically chosen to go deep, while the Mid-Bass can be specifically chosen to reach up higher into the midrange. Thus making the overall design easier.
With in a certain perspective, the old school 3-way - Woofer, Mid-Range, Tweeter - designs were the easiest, because each driver is well within it working range. In new school 3-way - Low-Bass, Mid-Bass, Tweeter - matching components is more difficult, but still workable. The hardest design, relative to picking the drivers is a 2-way design. It is hard to find the right compromise between low bass and good midrange. But when a good 2-way design works, it usually works very well.
Last edited by bluewizard; 11-04-2016 at 03:53 PM.