I know you isolated it by unplugging your RCA cables but I still bet you have a grounding problem. Are you using a PAC interface to the stereo? Where are your grounds for your amps? They should be no longer than a foot if possible and grounded to bare metal. If you are using a PAC adapter, you may still want to ground the head unit to the chassis as close to the HU as possible. You can also try putting a "ground loop isolator" in line with your RCAs. Search on Amazon and you will find.

 

Do you notice the hum at all volumes or just when your turned up halfway or more? The harder your drive the amp the more distortion and noise it will make. Also make sure your your speakers aren't wired for to low a impedance again causing more distortion. No offense but pyle is basically junk. in car audio your only as good as your weakest link and the pyle is likely that.

 

I bet it is a ground loop issue, you can buy a ground loop isolator and that will fix the problem.
Amazon.com: Raptor GL15 Ground Loop Isolator: Car Electronics