I was going by the assumption that we weren't going to be messing with atmospheric conditions enough that we'd have to break out the ideal gas law (though I suppose one could figure out how to implement underwater listening if that would improve the listening experience somehow... maybe sulfur hexafluoride would be easier)
Nitrogen bleeding pads, of course.:)
The point being that modding takes on a very few complexions if you pull back and look at it as a system. Acoustic reflection or absorption, or changing the medium density whether it be with cotton (ick) or some more resilient material or by increasing pressures before or after the driver. In essence, actually tuning a driver relies on being able to alter the speed that sound travels.