I am familiar with them. Like I said, they may very well be rigid enough to make an enclosure, they may not be ideal for a subwoofer enclosure due to their resonant properties. If they resonate in the audible range then they will not be very accurate for audio use.
I've actually seen subwoofers made with these tubes before. I would say if you were using less expensive drivers and electronics then go for it. For all I know they might work just fine. FWIW my tube sub despite 1" thick walls vibrates more than I anticipated. The plans given on GR's website are for very stiff enclosures. There's an alternate "sandbox" version which has a box within a box with sand-filled walls that is so dense you can't feel ANY vibration from the cabinet.
I can certainly appreciate saving on labor and materials but if you look at some of the best speakers out there you almost always find a well-braced and very dense cabinet. There are some aspects to physics you just can't cheat.
Of course you can try it and see how it sounds. If it works, let me know. Maybe try the tube within a tube and fill it with sand in between. So long as you have the correct volume you're fine. And get some poly-fil for the inside as well. About a pound should do it.
