If your A/V amp has front left+right pre-outs,then best/safest way to to it is as follows......
1) The front left and right preouts of your A/V amp go to a line-input on the stereo amp (aux for example).
2) Set the volume of the stereo amp to about half way(12 'o' clock )and run your speaker calibration routine on the receiver. Then always use this volume setting on the stereo amp whenever you watch AV - overall volume is handled by the receiver.
3) When watching AV, have both amps on and select the input that your sony is connected to on the stereo amp(aux for example).
4) When listening to music,only the stereo amp needs to be on, so you select the source thats required(cd,tuner for example)
Hope that helps