I remember many oodles of years ago, Bob Crump recommended those Pass and Seymor outlets, but warned everyone that they required a month of break in. Nobody believed him, I mean what could possibly be breaking in? He actually went so far as to describe the changes he heard over a 30 day period.
So I got some of these outlets he recommended and low an behold, the sound of my system was all over the place for about a month or two. Great one day and flat and lifeless the next, only to be followed by shrill, then boomy, then great, then crap again. Finally everything evened out and the system sounded good again.
That experience convinced me once and for all that break-in is a real phenomenon.