my amp has a knob to adjust the crossover frequency, what is this feature for and how can i tell if it is at the optimal setting. please reply thanks

 

The lower the crossover point, the less music there is going to your subs (and the more music there is going to your mains). The higher the crossover point, the more music there is going to your subs (and the less music there is going to your mains).
Keep in mind that setting the crossover point too high (anything much higher than about 80 Hz) can result in the localization of bass frequencies, which tends to diminish any imaging/staging you may have had.
There really is no "optimal' setting - its all about preference. Try out a few different settings with your favorite tunes and stick with the setting that sounds the best to you.
Bang on.

 

This is not entirely true. The crossover filters out certain frequencies. If you are using the amp to power subs, you want to use the low-pass filter. The numbers usually go from something like 20Hz and up. The higher you turn the LPF, the higher the amount of frequencies which are able to pass. (eg If you set the LPF to 120HZ, it will play the frequencies fromo 120Hz down to around 20Hz). High pass filters work the same way, but opposite direction (if you set the frequency to 120Hz, it will pass the frequencies from 120Hz all the way up to about 35kHz)
Look at your speaker specs and see what they are rated for. Tune to your liking

 

If you have a 12" sub, set the crossover to about 80Hz. If you have smaller subs, raise the crossover frequency. If the frequency is set too high, the bass will sound "sloppy".