umm i have this problem too, people on the forum told me it could be alternator/ or if you have am internal amp that came with car. i am just gona rewire everythjing and get a new alternator later with in the month.
well if you changed the rca's channels at just the amp... and it's still the same speaker... the it's either the speaker... wiring ... or amp... not headunit related... so try 2 reground the amp to a really good spot... **if u take the ground off you need to disconnect power first... take the fuse out of the power wire**... and if that doesn't work... put a new amp in to test if it's the amp...