I'll dig up some links later, but here's the quick answer.
Speed of sound is about 1130 feet per second. You can determine the "wavelength" of any particular frequency by dividing 1130 by the frequency, ie the wavelength of a 250 Hz signal is 1130/250 or about 4.5 feet.
If you have two signals of the same frequency but "out of phase", ie 1/2 wavelength apart, you get cancellation. If the signals are in phase, they add. You can actually simulate this pretty well by tying a rope to a doorknob and wiggling it to make waves, then tying another rope at right angles near the doorknob, having someone else wiggle that rope, and watching what happens when the waves combine. Or you could trust us
So... if you have a dip at 250 Hz, you're looking for 1/2 wavelength at 250 Hz or 2.25 feet. This can either be a round trip distance (eg. center of the speaker is 1.125 feet from the side wall) or the difference between two paths.
Once you understand this and start looking at all the different combinations of signals you can get, it's natural to panic and cower on your couch convinced that you can never get decent sound with all those signals interfering and reinforcing
EDIT -- forgot to mention that one of the most common causes of a dip like that is cancellation between main and subs at the crossover frequency. You didn't say what your crossover frequency was and I imagine it is closer to 80 Hz than 250 Hz, but some receivers *do* have very high crossover frequencies and I just thought I would ask to be safe.
