Advice, big problems

I'm 99% sure that the problem is my factory amp. I have the stock cd player and ran the rcas off a line converter connected to my front door speaker, it was just powering a sub and at most the amp put out 800 watts. I had no idea what i was doing at the time and used 4 guage wire instead of the recommended 8 guage. Dont know what happened exactly most likely the power and ground touched because the wire was too close but when i turned it on the system shorted. The strange thing is only my front door speakrs work now, not the rear door speakers or the speakers in the back. I took it to music in motion and they told my my factory up had blown channels. I would take their word for it but it seems like they were trying to take my money because they said in order to fix it i would need to purchase from them  several things that i didnt need, such as a new amp and new speakers. The speakers work fine there is just no signal getting to them. I tried using the rcas into a less powerful amp and running the speakers directly off of them and that works except when the car is on i get a whining noise with the rpms. I dont think it is because of my rcas being next to the power wires because  my  battery is in the trunk, where the amp is. I have heard that buying a new cd player would fix the whole problem, but i dont want to do that unless i need to, the car won't be mine for more than a year. I thought that maybe i get the whining noise because i am using a low leval line conerter to get the audio signal. I only get the whining noise when the high pass or full filter is on. Any help is appreciated and if you need more information just ask.

The reason the 4 guage wire was a problem is because it didnt fit correctly , and i think it shorted because the ground touched the power wire. Not completely sure thats what happened becaue i wasnt looking in the trunk when it happened. I know that blown channels in the amp is a major possibility, and when i said amp i meant the amp i was using to power the subs not the factory amp. Also, i hooked up the speakers to a different  and they worked fine, but i have new speakers in the rear doors anyway. Thanks for the tip on power wires, its obvious but i never thought about it, i know for sure that there is power carrying wires where im picking up the audio signal from.

Since replacing the HU won't fix the problem, do you know anything besides replacing the factory amp that will? Thanks alot that does clear up alot of questions for me.

I didn't know Delco used anything with that low impedence- wow.  Yes, that's definitely a heavy load for any amplifier.