I still do not see how different subs would not be able to correct a problem, with regards to null and peaks in a room, unless the anomaly was occuring at point that was outside the range of one sub.
Ideally, with two subs, I would probably prefer to have one in front and one in back, or one along one side wall, but not centered.
And what is a 'better' sub? More dbs, smoother and/or more accurate response, or just one that costs more?
Although, I can certainly relate to having the one (if they are different) that will go deeper and put out more, handling LFE.
I am very interested in the subject, as I mentioned I would like to do the 'right thing' for my own setup, and would be interested in the experience of others also.