Okay, I think I have this figured out, but if someone could verify this would work, and answer one question, I'd appreciate it.
I am going to install my 4 new speakers soon running off the stock head unit, and shortly I am looking at purchasing a 4 channel amp to power the 4 speakers, and then a mono amp to power a sub.
So, I'm planning on using a Line Output Converter to take the speaker signals down to a pre-amp level, and running those to the 4 channel amp. If I get an amp with a pre-amp out on it, and use this to run to another mono amp to power a subwoofer, will the volume be controlled on the subwoofer the same as on the 4 speakers controlled by the first 4 channel amp?
Where does this pre-amp out (passthrough) get the signal from, does it get it from one of the channels on the 4 channel amp, or is it a sum of all the signals? essentially, how does the balance and fade affect the mono amp?
Below is a diagram of the setup I'm planning on using: