Masa and Peter, this involves room boundary reinforcement and cancellation(not standing waves)which was discussed a little while ago from the standpoint of the reinforcement in low bass a sub gets in the corner. This involves cancellation a bit farther up the spectrum in the upper bass/lower midrange around 200hz. Roy Allison is noted for his research in this area as much as 30 years ago and the suckout of as much as 11db which can occur around 150-200hz when a speaker is placed equidistant from three near boundaries is occasionally termed the "Allison Effect". Some of the Audio articles I previously listed in the sub discussion also go into this, especially the ones by Allison and Ferstler.
Peter, I don't know how noticeable the improvement might be(there wouldn't be any at all if the speakers happen to be at unequal or long distances by chance). I'm sure you remember that I previously cautioned you that you and wife's ears had to be similarly positioned for best results.
