hi im trying to figure out whats the best frequency for my amp, i have two tens and i listen to mostly rap, i heard a really low frequency is good to have for rap but how low is too low? and where should it be. oh yeah i have a rockford fosgate amp and i know the ideal is between 50 and 100

 

depends on what your other speakers are, the box size and style, where it's at in the car..... play with the settings and see what sounds best to you (always change settings at low volumes to keep from damaging anything

 

Of all of the high-tech instrumentation grade measuring instruments in the world there has never been one made that can tell you what sounds better to you than you. It is your sytem. it is your ears and your taste that define what your system should sound like.
If you are competing then you have a different agenda. But if you just want your system to sound good then go with your ears and the ears of the others in the car. Adjust the cross-over frequency until you like it as long as you stay within the boundries of your components .
As you drop the x-over frequency lower that will make the driver that handles the frequencies just above it have to cut off at a lower frequency. that isn't good if you go out of the upper drivers range.
When choosing the x-over freq of your sub it isn't the higher frequencies that the sub can handle as much as it is the lower frequencies that the midbass or the midrange can handle( the driver that handles the frequencies above it) on it's bottom end.
If you set the x-over freq for your sub to 80Hz at 100Watts then the midbass or midrange also has to be able to reproduce 80 Hz at 100Watts without blowing the cone out of the controlled magnatic gap of the driver and without overheating the insulation on the wire in the voicecoil within it.
And it has to do it accurately.
Find the lower frequency limit of your midbass or midrange in order to decide the x-over frequency. This will be your lowest x-over frequency. Never go below this at the power you are running and your equipment will live to play tunes another day. Above that frequency your equipment will live longer. A higher x-over point will reduce strain on your components.
:-)