Hi.

I have a modern Pioneer Head Unit that feeds the front speakers (door and
dash/tweeters), back speakers and the RCA out goes to a PPI 2120 amp and
then to a JL Audio 10" sub.

Hence only the sub is amplified.

What I need to know, is to what X-Over Freq 20Hz-5kHz should I set it since
it is only feeding a sub. And also, for the X-Over type, should I choose
FULL/HPF/LPF ?

I just want to understand how X-Over are supposed to be set.

Thanks.

What I need to know, is to what X-Over Freq 20Hz-5kHz should I set it
since
it is only feeding a sub.

You should probably set it somewhere between 60 and 120 Hz, whatever sounds
the best to you. If you set it too low, there will be some music missing as
your front speakers can't reproduce the area you cut out. If you set too
high, then your sub will sound like ass. Kinda overbearing. Basically play
with it until it sounds right to you on a variety of recordings.

And also, for the X-Over type, should I choose
FULL/HPF/LPF ?

For a subwoofer, you'd choose the LPF, which stands for "Low Pass Filter"
This means that the crossover will pass all frequencies below the point you
choose and block all frequencies above that point*

HPF stands for High Pass Filter and of course is the opposite of above

FULL is no crossover function--basically OFF.

*Crossovers aren't exactly "brickwalls". There is a slope or rolloff in
frequency response. If it's 12dB/octave for example, that means that the
frequencies above your crossover point will fall off by that amount. e.g. if
you choose 80 Hz, then 160 Hz will be 12 dB quieter, 320 Hz will be 24 dB
quieter, 640 Hz will be 36 dB quieter and so on. If it is 24 dB/octave and
you chose 63 Hz as your crossover, then at 125 Hz you'd have a 24 dB quieter
sound and at 250 Hz you'd have 48 dB quieter sound.

-Bill