can you elaborate this a little more. I understand it but not really how you say it. If that makes sense.
In car audio you had to deal with crossovers too. Imagine this:
You have a Pioneer deck and a nice Alpine amplifier powering some subwoofers. The Pioneer is going to power your front and rear speakers. The Pioneer has a built in HPF (high pass filter) that is adjustable in the range of 50hz, 60hz, 80hz, 100hz, 120hz, and 150hz. What that HPF does is filter out the lowest frequencies from your 6"x9" and 5.25" speakers in front and sends that low frequency information out through the subwoofer preouts on the Pioneer to the Alpine. Because the Pioneer is performing the function of crossover the Alpine's x-over should then be set to the highest setting so it's roll off does not multiply the roll off from the Pioneer's crossover.
If you look at the graph below it will short of display what I mean by roll off.
As you can see the red and green lines dip down. The frequencies don't just cut off, they blend together.