I find pluging an MP3 player into the amp helps trouble shoot if its a signal wire or a ground issue.

I have never myself seen RCA`s placed to close to the power wire cause any whine but who knows.
Bent, twisted or pinched RCA`s now that I can say I`ve seen cause it.

Sometimes that whine is due to a bad ground at the HU, not the amp. People spend days re-grounding there amps, switching cables with no luck.
Best advice I ever got was to ground the HU to the amp, just run some 16 guage from front to back.
Don`t knock it til you try it