Sulfur hexafluoride. It's not cheap, but it's commonly available because it's
used as an insulator in HV electrical systems. Completely non-toxic, but heavier
than air, and therefore suffocating.

I was about 27 years old back in 1977 when I first heard these
speakers. They were extraordinary, and created a holographic
transparent soundstage which I've yet to hear reproduced.

Unfortunately, I've heard that reliability is not one of their strong
points, and they use a very unusual gas in sealed bags...
(something) hexaflouride. Might be difficult to find if needed for
repair. But if someone is enterprising and finds a quality pair of
XG-8's, they're in for an audio treat.