I've noticed that this does not happen, really, with Dolby Digital movies. But I have noticed this considerably in some DTS movies. A great example is in the ring drop scene in the LOTR: Fellowship EE in DTS. If you cut your side surrounds from large to small you'll notice a further extension of the bass and the "wave" disperses. All in the low end of the hertz spectrum. A fascinating experiment really.

"We're on the island of Misfit Toys"