When you put stuffing in a box it does two things:
First of all, it introduces an often desirable resistive loss and secondly it increases
the apparent volume of the box. In this second part thermodynamics comes in:
In a empty box, you get a adiabatic process, meaning that when the cone move
inwards it raises the box pressure and hence the temperature. A gas with increasing
temperature tries to expand which will cause an increased suspension stiffness and
raised resonant frequency as a result.
When adding stuffing you move toward a isothermal process, the stuffing cools the
raising air temperature in the box and give a less increased suspension stiffness then
in the adiabatic case, and a lower resonant frequency as a result.
In other words, yes the prefix is thermo, and it got everything to do with it!
Hope I made myself understood, my english is a little rusty...