Alberto, John explained it very well(I often use 560/one length as being a bit easier mental calculation)and you can find all sorts of info, including calculators, by Googling a topic such as "room modes". An example is
this article.
A factor totally separate from room modes leading to peaks are suckouts caused at 1/4 wavelength from a room surface or 1/2 wavelength between drivers of separate speakers playing the same frequency.
So, what we have are players and amplifiers(and even wires!) doing an essentially flawless job, and speakers such as Axiom which are quite accurate in their response, but then the room screws things up.
