My experience with pre volt out is I bought a head unit that had no pre volt out, had an external amp say 300 watts or something with a 12 inch sub. used it for oh say a month. I bought a new head unit with a 2v pre out, and blew the sub out within a day using the same amp. Without those pre volt out, the amp alone did not have enough power to blow the sub out.
The pre volt outs provide a ton more power and the amp further steps it up. You can basically take an amp, double the voltage out of the head unit and increase the power in the amp, by alot. Enough to destroy subs.
So 2v vs 4v, you will prob need less gain on the amp using the same wattage to get the same results.
Thats it, now the question is, is it worth it for the upgrade? No its prob not.
Also regarding the ipod cable, go for the apple composite cable instead, same exact thing, much better build quality and standards. A single unified cable instead of the black split cable right at the head with shrink wrap. Also the head connector itself seems a lot better build quality, the pioneer cable I had you can actually see how cheap the quality is, its light and day. And they are the same price, imagine that.