what's difference does it make if your head unit has 2v or 4v pre out? i am planning on going to a 4ch amp for the interior speakers, and a bridged 2ch amp for the sub, but will the 4v preouts make a big difference or not?

 

yes. for the most part, the more signal you give the amp, the easier it is for the amp to produce power... you'll also cut down on interference (noise, etc.)

 

Certainly, it's always nice to have some extra voltage, but I don't really think it's going to make a drastic difference - so long as you have good amplifiers you should be fine. You will have to crack the gains a bit more for the amp to make power, but, with a good install, you should be pretty noise free.

If you feel like you need stronger oput and you already have the headunit, you can always pick up a line driver and up the voltage.

Virtually all of Alpine's 06 line is going to be 2V, so if you like Alpine's, you had better get used to it

 

That sucks, my Kenwood is 5V with the option to turn the amp off to furthur enhance the sub output.