On the way back from my native place in Rheingau I went via Schwäbisch Gmünd to visit Nubert, a company focused on audio speakers. While my initial research was focused on the nuLine 24 or nuBox 483, the excellent and attentive staff helped me to find a pair of speakers to use with the old Braun Regie 520 I brought over to Munich. After quite some thorough hearing tests with different kinds of music in two of their sound test studios (first the small ones, later the large ones). I finally decided to go for a pair of the nuLine 34 which provide me with great sound and are still able to be powered by the old receiver without issues and the difference to the nuLine 24 was quite impressive.
At the end they even gave me free adapters for the audio cable to connect to the old Din sockets in my receiver.

Jeep in front right after picking up the new speakers:

Check the nuLine 34 at Amazon:
Nubert nuLine 34 – Regallautsprecher (Stück)