Does the VU meter on audio amplifiers measure the input or output level?
A VU (volume unit) meter is meant to show the signal level, not the amplified level. If it measured the output level, it would be almost useless at any volume setting other than maximum.

A VU meter is meant to show when audio signals clip, so you can reduce amplitude. This is especially important when mixing and mastering audio, as you need headroom for post processing effects like reverb and compression.