Do the other speaker manufacturer's have a forum, similar to this one? I've not heard that they did. I would also imagine that if they did, the owners of those particular speakers would have a bias towards the ones that they own, just the same.
I am sure there are others and practically speaking, you are totally correct in that the individual forums, such as this one, are going to be frequented by those that like the brand and of course, for the most part, own it. As I see it, the problem that has all too common existed on AVS is that nowhere else have I seen the viciousness and hatchet jobs along with a confrontational attitude towards other brands that the vast majority of time, the person doing the criticism, strangely enough, has probably never owned or even heard! It seems to attract a certain culture that, rather than provide meaningful discourse and information, prefers the above and I am not exactly sure why other than the fact that it seems the administrators obviously wish to have it that way.