This has been something I have always wanted to do as well. A real qualitative subwoofer comparison would have to be done outside (or in a room as large as an indoor stadium). Also, as you say, the equipment would have to be hidden. That is the only real way to fairly compare subwoofers, at least with human ears. They would have to be compared using fast A/B switching between the subs as well, at least for complex signal content like regular movie or music content. The problem is, GTG's are all about having fun, not rigorous testing. I wouldn't want to ruin a GTG by insisting on rigorous adherence to a strict testing regimen. GTG's are usually about drinking a lot of beers and seeing how loud some speakers get, and that is fine, but this kind of testing wouldn't work for that kind of gathering.
I would want to see if these subs can be distinguished when they are operating well within their linear ranges. SME seems to think that there would be a difference. I don't think there would, but such a test, if conducted appropriately, would certainly serve as good evidence one way or the other. Neither SME or myself would be able to properly participate, because we already have predispositions that a blind test could not overcome. Still, I would love to hear it for myself.
I need to secure some kind of grant to do this with.