I will K.I.S.S as much as I can.
X-over point is basically the freq point that you choose where the speakers audio starts to give its own information to the subwoofer.
The first reason why we even use one is because TRULY full range speakers are extremely rare, and extremely expensive.
The second reason is room acoustics, and the way responds in a room when placed in all those corners. There are other reasons too Im sure.
So, your speakers and your room acoustics, and setup.
Usually people x-over at 80hz. This one commonly works well. However, once your speakers start getting small enough, you need to put that xover up higher. The MAIN issue with this is that the sub becomes more and more localizable as you do this. So having the sub near the mains could ameliorate such an effect. It all depends, includeing where you are sitting.
Really the easiest and best way is to use your ears. The louder you listen the more likely you will want to raise that xover.
Now a xover is not a brick wall. There could be a LOT of info being shared. Its all about the "slope". 1st order, or 2nd, 3rd, and so on. Which basically means 6db/octave, 12, 18 and so on. Where it gets extremely technical is with the types of filters, whether Butterworth or Linkwitz-Riley, etc.
To make you more confused, different receivers can have different slopes. My guess is that most receivers are 12db or 2nd order. Sometimes they will be like that and be 1st order/6db for the speaker outs, which would strike me as strange, until I learned that they are depending on the speaker to have its own natural roll-off already.
Higher end receivers, pre/pross sometimes come with 24db (4th order) slopes. I think of NAD and Cambridge for example, two brands I own. This is a steeper rolloff. What that means is that less info is being played by BOTH speakers and sub. More close to a wall, though a wall it is not.
Therefore, the higher the order or slope, the less audio is being shared by sub and speakers.
Hope this helps.
Now that I have completely freaked you out, I would try 100hz for starters on your system. 80hz might work, but again it depends, and really the only that matters is:
Your Own Opinion
Can any one explain me what is cross over setting in an AV receiver? How this work. I have Onkyo 805 receiver and KEF 3005 5.1 speaker system?
Give me some tips do some research with cross over setting.