If you have Windows, can you right-click on the volume icon, then select "Open volume mixer". (similar but diff I'm sure for each OS)
Then you should see what levels are set for different system things, including your external or Pianoteq soundcard audio.
If that is raised to max there are still other things to try below - but if not, it may be the one setting you never think to find.
I have my external audio at 98 (seems it was that way by default - happy, so left it). Seems OK - and I tend most volume aspects on the outboard hardware knobs.
Pianoteq has in years past, seemed a tiny bit too loud to me, then a little quiet, but now seems a good sensible level (when I know things are set similarly in that Win audio mixer). No plugin should sound way loud compared to others, and users hear 'too quiet' as lacking quality etc.. but I think if our settings are OK, Pianoteq now seems to output safe yet high levels within a very pro-happy-making range.
Also - turn off any PC or OS installed "wide FX" kind of extras.. that may be achieved while checking your audio like above.. but if you open that mixer, then left-click the icon for your device on the mixer's top row.. that will open settings for the device where you can "tab" to various settings from volume and turning on/off whatever junk and good stuff your system has installed.
On "Enhancements" tab for example, I have check-boxes for "low freq protection" "virtual surround" "room correction" "loudness EQ" - any of those could make Pianoteq or other plugins sound wrong wrong wrong.
On a "Spatial" tab there's a selection of "Windows sonic for headphones" - honestly.. probably OK for watching movies?? but these kinds of things switched to ON can make music tools sound very weird and wrong.
Good luck - hope that's just the thing!
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors