Pretty much forget a pre amp at this point unless you wan't to go to separates, meaning amps to drive the speakers, a pre amp for control.
A receiver is like an all in one computer. It has a tuner, pre amp and amp. Not an eloquent solution due to the amps are usually average at best but if performs well with the speakers you have, it isn't a bad choice.
Depending on the receiver, it can send out a signal to an amp or amps(or powered speakers) to relieve it's self of trying to drive 5 to 6 speakers.
"so with an amp is it possible to do something like PC---->DVD Pioneer home theater-----> Amplifier -----> 2 front speakers / and for the rest DVD Pioneer home theater-----> Sub+3 other speakers?" So yes it is very possible. But more like if you can and the go analog from PC and analog, if you want a separate amp to drive your fronts. It would be PC>Receiver>amp or amps/or powered speaker(s) input and the rest would be powered by your receiver. Otherwise it would just be PC>receiver with the normal speaker wires to your speakers.
Without knowing the the model of the receiver and/or if this is a home theater in a box setup, I really can't give you anymore advice.
To give you an idea for one of my setups I have I used both HDMI(from a video card and blu ray player) plus I also use analog from my SBZ(like said above, some reason mobo chip is bad) and use this surround processor pre amp.
http://www.outlawaudio.com/products/975.html.
be seeing you, the Prisoner...