Try different size caps for this as I am not sure what frequncy range that this noise is in.
Try a 1uF ceramic to start with and just put it from +12volt to ground, if that don't work double it to 2uF, if that still will not work double it untill it does.
I am wonerding if this is an internal noise problem. How is your RCA's Filtered??
Using a choke will cause alittle more resistance to DC, causes a ground loop feedback to go through positive forced to find a loop tied to negitive, so if you use a choke use a cap behind it to form a signal or noise ground. Yes chokes oppose AC or noise riding on DC, but care must be taken or more worse things can happen.
RCA's are coaxial cables with a ground referance shield. If I had a chance I would O-scope the +12 volt, the Ground referance on the RCA's, Ground going into the amps, and the speaker outputs with no input, then find out what frequncy the noise is in otherwise it just a shot in the dark blind folded.