Does the VU meter on commercial audio amplifiers measure the level of the input signal or the level of the amplified signal?
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.
I use VU meters and they indicate to me the level at which clipping of an audio/music signal is likely to be occurring in the system/amplifier so, in your amplifier it doesn't matter where it is connected to, providing it correctly represents the potential overload point of your amplifier.
It is generally recognized in the music industry that the peaks of music are usually about 6dB higher than the average level (which the VU meter reads) and hence -3VU is appropriately positioned in the middle of the scale.