I just found the following quote:

"Only the woofer/subwoofer were not bipolar. These relied in the use of SF6
(sulfur hexafluoride) gas (which is inert), to increase the virtual volume of
the enclosure. As SF6 is an 'ideal gas', it operates as an 'isothermal'
spring, thus avoiding the problems with 'acoustic-suspension' loudspeakers
that operated partially as an isothermal and partially as an adiabatic system.
Some designers seemed to lave little knowledge of Boyles Law or the Laws of
Thermodynamics.

"In effect, the use of SF6, increases the virtual volume of the enclosure by a
factor of 27! As can be appreciated, this both lowers the distortion as well
as permitting a lower resonant frequency of the woofer."

It's here...

http://www.dayton-wright.com/WATSONLABS_.html

I should point out that the bass cabinet is not completely filled with SF6. If
I recall correctly, the gas bags were separated with foam sheets.