CPU and TB has nothing to do with levels. My hunch is that something is off somewhere else.
_- What do you mean by “wanted to do a null test, then analyzed”? which steps did you take specifically?
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- How large was the difference of TP and LUFS?
\ - What do you mean by “bounce”? Actual Cubase/Nuendo “bounce” or “export audio mixdown” (Pro Tools “bounce”)?
\ - You said after “without any plugins inserted”, did you mean first before the bounce you had plugins inserted and you tried without when you did the “bounce”?
\ - You said then “and the results where[sic] the same”; did you mean you still had that difference?_
Intuitively I’d say check your routing. It’s not uncommon for people to accidentally set up new systems differently and not notice. Your project should load the same, but it’s I guess hypothetically possible for your “studio setup” or whatever it’s called to be different and unaffected by the project. So check that you didn’t accidentally double up on outputs/monitoring paths etc. And check where you’re measuring levels.
Also make sure automation plays the same if you’re using it. And check all faders and trims etc.
Really need more clear information about what you did and how though.