8 subs in truck need your opinion.

what brand of subs, how many ohms 2,4,8 , whats the RMS rating?  It matters the type of sub because different brands of subs usually need a certian type of enclosure.  If they are required in a ported box vs sealed box you'll need almost twice the space for each sub.  You can't really choose an amp till you know what the subs require of the amp.   First, is the amp brigable, 2ohm stable is all most required if your going to have more than one sub.  If your going to add four 8" subs with an RMS of let say 200watts then each subs Max power handling going to be about 400 watts, that's 1600 watts all together.  For best results you'll want your amp that would be able to handle this but its not nessisary, beening able to comfortably handle the rms should be fine.

 Answering the questions that I've asked will help to give people who read your Post a  better picture of your questions and mabye give you some solid answers.

Good Luck!