Thank you for your spot on advice, Stl-986.
You are correct the JBL GTO 6528 are 2 Ohm (6529 are 3 Ohm). The infinity speakers are even sold as "build like 4 Ohm speakers due to cable and connectors" which will result in 4 Ohm.
In my case Pioneer doesn't even recommend using 2 or 3 Ohm speakers, but 4 or 8 Ohm speakers.
PROBLEM over time: HU upgrades!
I had many different HUs beforehand with 2 Ohm HAES and got the JBL 2 Ohm speakers later.
I noticed, that my latest PIONEER HU would get "pretty" warm, with my 2 and 3 Ohm speaker setup only. But after using an active sub with a sub RCA (can't find any infos on Volt output), the HU is way more relaxed and less warm, additionally I use pioneer's equalizer app to adjust each speakers input setup.
Long term I should and probably will switch to an overall 4 Ohm set-up. I just need to find the proper speaker sizes, which can be pretty annoying, as far as I remember.
For now the setup seems to work pretty okay due to the heavy help of the active sub.
@KevinH1990: Yep, I think I ordered that long wodden trim from Porsche directly (or was it sold on ebay as brandnew?), the other two were made by a specialist.