Yes, I did a little research and figured that most factors cannot be changed easily. So improving the factors I can change was my strategy: Damping damping damping
The bose cases have a volume of 6 litres. I gutted them completely of their interior and upfront plastic sound channeling. That possible gained a few extra ml of volume.
I moddified the speaker holding frames, so that they can hold the cross-overs and the woofers (extra distance layer). That possibly gained another few extra ml. The cases are still a closed system.
My mid/low HERTZ woofers => VAS = 8 l
Most other 6.5" woofers have a VAS of 10-13 litres. And going smaller, like 13cm...have max 4 l VAS.
They are (almost) perfect, but won't reach their full potential.
But that is fine.