I think that a simple plot like this is a really good start. This tells you how the subwoofer behaves with varying SPLs (this particular sub stays very linear) and you can see what its limit is, when it starts to compress the signal. This particular sub stays very linear and starts to hit its limit at slightly over 100dB. It also gives you a good idea of how large of a room the sub will play well in.http://www.audioholics.com/reviews/speak...view_fullscreen
Yes it is only a small part of the story, but many manufacturers only choose a plot a reference level, or some other randomly chosen level, so you don't get that extra information that the above graph in the link gives. Then again many manufacturers don't even given a frequency response plot at all, and we have very little to go on.