This is how my system is set up - works a treat:
Connect the AV amp "Main Pre-Out" connection to a spare set of inputs on the hi-fi amp.
Connect your main stereo speakers to your hi-fi amp as normal and your surround speakers and sub to the AV amp.
To set up, turn on both amps. On the hi-fi amp, select the input the AV amp is connected to. Set the volume control on your hi-fi amp to about 12 o'clock. Run the test sequence on your AV amp and using the AV amp controls only, adjust all speaker settings to the desired levels.
DO NOT TOUCH THE VOLUIME CONTOL ON THE HI-FI AMP.
Adjust the volume controls when viewing using the AV amp volume control only.