You lazy bich....

j/k
OK..
Anywhere on the chassis is an acceptable place to ground.
This is what a modern chassis looks like
Any clean bare spot will work.
If you are having noise problems and changing your ground does not help, try rerouting your RCA wires, they should run as far away as possible from any power wires be them for the car or for your amps. If they must come in contact make them cross over in an X fashion, you never want RCA's and power cables running closely parralell.
Also check the health of your ignition system, a plug wire with a chunk out of the outer cover will radiate noise, also crappy plugs can do the same, When was the last time you checked everything out under the hood?.