German wikipedia provided something interesting. The resonance effect is working best from 0,75*fb to 2*fb (resonance frequency of the enclosure). Subwoofer normally operate from approx. 20Hz to 80Hz.

The previous owner tuned the enlosure to 27Hz, to work nicely from 20Hz to 54Hz. It would have been interesting to adjust the crossover (80Hz, 4th order in my case) to 54Hz and to run it as a deep bass addon with my living room stereo. But since I’m running it with small satellite speakers on my desk 27Hz – 72Hz seems like a good choice…