With some more info and maybe some screen shots of your Cubase/WaveLab and interface mixer settings somebody might be able to help you.
It sounds like a software configuration issue.
I use Pro Tools and WaveLab back and forth all day and the levels are the same from both. -12dBFS is the same from all my software to my monitor controller. With the right settings, your levels should match too.
My initial post was thinking if you had a bass track recorded in Cubase, and the mixer fader for the bass track lowered below 0dB/unity, and then you bring the WAV of the bass into WaveLab for editing…WaveLab by default will play that file at whatever the level the file is recorded at (no fader level in play), so if you’re lowering the fader of a specific track in Cubase, you could see a large difference.
But, the master output of both apps should be identical with the right settings.