Before you go playing with the xover's and all that, follow my steps to setting your gain. That will set the foundation for setting all the other stuff.

 

no, the top dials are the gain controls, one for channels a/b, the other for channels c/d

Input level aka gain does not mean how much power the amp is putting out. The amplifier gain is there to match the amp to the signal voltage of the source or to another amp. In this case it's to the head unit, if the head unit has 4v preouts then you want to set the input level aka gain to 4v. The idea is to properly match the output from different pieces of gear so that each communicates the cleanest signal to the other, resulting in maximum performance and minimal noise & risk of damage due to clipping.

 

when you were saying about the cross overs on the amp that i am useing for my speakers i have the HPF to cutt off at 75 so on the amp for my sub should i set the LPF to 75

on my hu it has 4V outs but on the PDX1.1000 you can switch from 1v-8v or .1v-1v but it plays louder at the .1v-1v?

 

There is no magic setting and I surely can't tell you where to set your xovers over the internet. Far too many variables come into play when setting xovers, I have no clue what your system sounds like, where it's deficiencies are, what's it's capable of or anything. The best thing for you would be to listen to one of your favorite cd's on a nice home stereo system. Really listen to it, listen to the bass, the mids, the highs, listen to all the sounds and instruments in the music, listen to the clarity and depth of the vocals, familiarize yourself with it to the point to where you could bring that cd to your car and hear what's missing. Are the highs too high? Is the bass too muddy? Does it go as low as it should? Are you missing a wide range of bass that should be there? Do the vocals sound overpowered or muffled? Can you hear all the nuances in the music that you heard on the home stereo system? It's going to be a challenge to get your car system to sound as good as a nice home stereo system but at least you have something to aim for, you know what it should sound like and you can do whatever is needed to get as close to that as possible.

The fact that your system plays louder at the .1-1v gain range setting doesn't surprise me considering you don't know how to properly set your gains or seemingly any other setting in your system. Having it set there when your HU puts out 4v is only making your amp work way harder than it needs to. What you're asking your amp to do right now is to amplify the input signal 4 times more than it really needs to and since you have a decently strong signal going to the amp already it's no surprise that it's louder at that setting. You're over working it! I guarantee you that you can't turn your system up 3/4 of the way, it'll start distorting at or around the halfway point, if even that.