There are matters of compatibility between a separate preamp and a power amp that don't occur in an AV receiver, where the engineer/designer knows in advance what the gain of the amplifier section will be and how much output (voltage) that the preamp section of the AV receiver will need to deliver to the amp section to allow it to reach full output.
When you run a separate preamp or the preamp section of an AV receiver, it's possible to run the volume control of the preamp too high, driving the preamp output into distortion before it reaches the separate power amplifier. Although preamp outputs should be standardized at about 1 volt, some preamps may greatly exceed that. I once measured a Sony device that put out 18 volts at maximum rotation of the volume control. That sort of output would drive the input of a power amplifier into distortion. The latter is not the same as the output of the amplifier clipping.
It's just overloading the input circuit of the power amplifier.
What is the setting of the volume control on the preamp or AV receiver when you are driving the power amplifier?
Axiom Resident Expert