Also the lower your tune the filter/port the more acoustic energy will be shifted away from 25 Hz, then to 20 Hz then to 16 Hz. Since there is more audio at 25 Hz than the lower frequencies, some people prefer the 25 Hz setting for more bass. If you have a parametric equalizer for the bass frequencies, you can alter the bass output at different frequencies to get 16 Hz tuning with the 25 Hz bass emphasis too. If you don't have a parametric equalizer you just have to experiment with the three frequencies to determine which you prefer. To me, unequalized, there was little bass output. At 20 Hz I started getting regular bass output so I stopped there.
