Installed Aftermarket Headunit With Mach460 - Volume Is Now Very Low..

I had the same issue when i replaced just the HU, ending up having to buy a 4 channel amp,speakers(factory use 3ohms), and just re wired the whole system.

Did you hook up the amps? You should have 2 wiring harnesses...
Look up my recent posts regarding my new Pioneer.

That Mach 460 system seems to use a unique input level in that the line level input is too 'loud' and the pre-amp input is too soft.
Use the line level input instead of a pre-amp input that you are using now. You will likely want to use a reducer to get the volume level down to a reasonable level, or you won't have very refined volume level control. You should be able to buy a reducer from any reputable auto sound store (likely out of luck at best buy or equal).

Having said that, I said screw it all and run an alpine head unit to alpine amps with boston acoustic speakers and monster wire connecting everything together. A single 10" alpine sub round out the system nicely.