The 20 vent hole pair is the one that comes installed on the headphones and it mechanically tunes up the bass.

To install the 4 vent hole cushions retract the ear cup/headband extensions and remove the installed cushions.

The Beyerdynamic DT1990 is supplied with 2 pairs of ear cushions, one with 20 vent holes for bass boosting and one with 4 vent holes for best bass detail.

If you have tone controls and want more bass detail you then install the 4 vent hole pair.

The only thing of interest here is that the B&O H6 is good for escaping the recording studio's acoustics and the Abyss