Loudness is not just peaks, but relates to average level usually measured in RMS (root mean square) or more commonly now LUFS (loudness units in full scale).
The levels inside the mixer will not be exactly the same as the original file. Inputs are ‘padded’ to bring their levels down so you have headroom. Commercial tracks often have little to no headroom. You must remember you are operating within a digital signal processing medium with a maximum hard brick wall limit of 0dBFS.
The VU meter zeros are the NOMINAL recommended peak level for the mixer only with continuous or near-continuous signals. This is actually to provide sufficient headroom for, in contrast, very dynamic audio that will instead need comparatively higher metering levels to reach the same perceived loudness as the near-continuous stuff while still having reasonable available headroom to prevent all clipping. Audio with a high average level will have its peaks and average closer together and therefore the nominal VU metering point for something like a sin wav or pink noise or Sonic Youth is going to basically be at or close to where its peaks show on the meter. In other words, you don’t get any bounce on a responsive VU meter with a sin wav, and there’s no confusion that its average and peak is the same thing and ought to be metered at about 0dBVU if you’re trying to get loudness about the same from track to track. A dynamic track is going to bounce below and above its average point, and should therefore have troughs below nominal and peaks above it when the music is in full swing.
The zero on the master out volume control is the UNITY – that is the point where the output for that particular stage is the same as the input level of that stage. So if you put the master knob at its unity, the level from the master stage will be the same as the level going into the master volume control. You should definitely have the master volume control at unity.
Nominal and unity are not the same thing. The meter zero is not unity. The zero on the master volume control is not nominal.
There’s nothing wrong with that Raw Denon Recording’s level and is what you’d expect from recording the record out or exact in situ internal recording with the gain structure you described on properly-designed digital gear prior to mastering it in a DAW or sound editor. Completely normal.
DJ as I described and in a DAW or sound editor in the future you can bump everything up by an even 6dB later. I advise you to not raise the volume in post by such that the final version is higher than -3dBFS.