First off do the Hi-Fi and normal audio systems power any speakers off the head unit amp?

If they do, a quick addon for better sound can be do disconnect the 12volt power supply to the head unit and run it directly to the battery with a inline fuse.
Something like
Just cut one end, attach with a butt connector to your head unit, the other end with a terminal to the battery...

And viola...better sound quality.

 

The non-HiFi powers the speakers off the headunit, and the HiFi doesn't (seperate amp).

Don't mean to poop on your idea, but I doubt powering the HU directly off the better will produce "better" sound. I know that the HU has other functions that would make leaving the factory wiring alone a much better option.

 

It does for many "upgraded" HU's..as what ever comes to the factory wiring harness isn't 12v more like 11 or 10 after going to the other dash functions

 

This one isn't upgraded though. The basic radio and HiFi are the same HU. The point I'm trying to make is that this isn't a standard DIN head unit. It's integrated, with other functions. You can program the digital display on the tach through the radio, for example.