ok i know higher preout voltage give u cleaner sound at high volumes, what does lower resistance do? also if I use a line driver to up the voltage, say from 2.5V to 4V, would that be the same quality as having a hu that preouts to 4V? I'm kinda kicking myself right now for getting a 2.5V preout when for $20 more I could've got a 5V preout. But it's too late to return the unit so I'm wondering if it's worth it to get a line driver and up it 8V or something.

 

2.5V is fine for most applications so don't worry about it too much. One of the most respected head-units ever made (Alpine's 7909) was only rated at 2V. If you have high-quality amplifiers you should be able to match the head-unit's preout voltage with the amplifier's input voltage with no problems. I doubt that you'll hear much of a difference in the same system with a 5V head-unit provided that everything else in your system is up to par.

What is your system comprised of at the current moment (head-unit, amps, speakers, processor, EQ, etc..). You might be able to spend the money in another area that will benefit you more than a line driver.