I don't know if I totally agree. To me whenever I have gotten a nice amplifier be it home or car audio, the amp makes a fairly large impact at lower volumes. Bass just always seems to tighten and music is fuller and more dynamic. Perhaps it is a wattage thing where it is just that instantaneous demand that is supplied without effort compared to a lesser amp or receiver not being able to provide just that nanosecond of high wattage. . .. I don't know but I have noticed the difference at very low volumes on the same setup minus an upgraded amplifier.
Just some thoughts
