The volume of the cabinet is a spring - while the property of the PR is mostly mass. The whole thing is merely a relatively simple resonant system. Of course there are losses and interactions and the issue of "Q" (the bandwidth of a resonance compared to its maximum, etc. But when you are all done it is a resonant system, that's all.
As far as filling with "gas" (other than "air") that's not of much use as the cabinets are awfully leaky, even when sealed to the limit of your ability to seal them! The only effect that changing the gas - at normal atmospheric pressures - would have is to change the spring which would effect the resonance and of course change all the parameters along with that. But they all tend to change when you change the mass and you change the volume of the box, the driver, the stiffness of the PR's suspension, etc. So, there's probably not much benefit, if any, to be had there.
The Dayton Wrights used gas as a dielectric, without which they would arc through rather instantly. Btw, they also used an awful piezo electric tweeter, so go figure!
_-_-bear
