I see what you guys are saying, but at these sizes, every inch of size increase means an even greater amount of weight than the previous inch. I could see a properly braced 24"x24"x18" going well over 100 lbs easy. However, if you sonotube it, probably wouldn't be that bad at all. My back is kind of sore right now, so it is painful to think about moving around a massive sub right now, but maybe I am just a bit too sensitive for that at the moment, lol.
If you were to sonotube it, you would need at least an 24" diameter sonotube I am guessing. You know what I would love to see, a dual opposed FTW 21 Sonotube, sort of like a extra large version of Bossobass's Raven subs. If my math is right, for that you would need a 3.8' high 24" diameter sonotube, would would be an approximately 12 cubic foot volume. That would be a very formidable subwoofer.