Looking for some advice please, I have a 2008 model with the basic sound system installed and I want to change the speakers.

I want to fit a 4ch amp and wire it to the speaker loom thats there and then to either 6.5 components in the front doors and coaxial in the rear. Or would it be possible to put HK speakers from a donor car into the doors as it would be an easier install.

Also would it be possible to wire an underseat sub into the system using the 4 ch amp.