Yeah I can't really agree that an AVR could sound good for movies but not music or vice versa. They're audio signals either way. To really compare two amps, you need to keep all the speakers the same, level match as closely as possible, and remove all forms of equalization. Then to add another layer to it, you can run room correction on both units and compare again. Also then if the amp has a special or proprietary feature that helps the audio, try that too and weigh it in.
But all extra features aside, with every other factor kept constant, if they sound different then it should be the case for any audio source.