As suggested by @wolfger it looks like you are getting confused about monitoring your mixdown in Cubase. To hear the reference mixdown you made, why are you setting the fader of that channel to -6dB?
To hear that mixdown at the correct level in Cubase you’d have to set your fader for the imported mixdown channel to 0dB (unity gain), and you’d also have to set your master stereo out fader to 0dB (unity gain).
As a test and to make sure you understand what is happening try the following:
- Export a stereo mixdown with the Stereo Out faders set to 0dB (unity gain). If necessary reduce the mixer channel volume faders so that the level on the stereo out meters does not clip. Above all DO NOT change the stereo out fader level. This MUST remain at 0dB for this test to work.
- After exporting, import this stereo mix onto a stereo track in the same project and set the faders for this track to 0dB (unity gain)
- Open the mixer and activate the phase button for the imported channel so it reads Phase 180.
- Start playback.
Upon playback, the result will be silence or near silence (depending upon what processing / plugins were used), which means the exported mixdown = the mix. As you may already know, this is known as a null test.
What happens outside of Cubase is another subject. There is a great deal of misunderstanding and confusion surrounding this topic. See: Why do Audio Mixdown .wav files sound completely different then Cubase stereo outs?