Thanks, jobo_ph. I figured that the wiring scheme I had in mind is not the most efficient way to go about things.

It turns out that my head unit has different equalisation settings depending on the volume which is for protecting the stock speakers. It also does not have a digital out. This really complicates things as it sort of becomes necessary to use an external volume control to avoid changes in equalisation. So now it seems that I should really consider an aftermarket head unit which significantly changes everything including what I'll end up spending.