how good will a 4-channel amp with 2 channels to the door speakers and 2 channels for the amps sound?

 

4 channel amp, 2 to the door speakers and 2 for the amps?
not following you.

Do you mean 2 channels to the front, and 4 channels bridged for a sub?
or 2 to the front and 2 for the rears?

using an outboard amp with aftermarket speakers will always sound 1000 times better than running aftermarket speakers with the internal amps on the deck

also, it all depends on quality of the amp and drivers.

stay the hell away from the SONY Xplode line, that stuff is pure garbage. It is cheap, it is crap, it clips like mad, and completly sucks my ***** on all sides.