Mystery of the Disappearing Frequency

I have only been involved in the pursuit for about 3 years(since my retirement) but have tried to read and research extensively. In this work I have not yet found a satisfactory answer to my dilemma. I have a 10 Db dip at about 50 Hz!!  This alone of course is not that unusual, what seems to be unusual is no matter where I take the measuring microphone(calibrated Omnimic) the dip never goes away!!  I have walked all around my somewhat small listening room(15 by 18 by 8) and the dip never get significantly lower. I have certainly read about dips but always thought they were specific to certain areas due to nulls and cancellation effects. My room is a little unique but I do not believe the following factors account for this dip. The long sides of the room have a 10 foot long bank of windows covered by thick drapes. It has a wood floor on a pier and beam foundation and the wall behind the speakers is the wall where the main house met the attached slab garage before the garage was converted to a few small rooms. The small amount of wall material is not sheetrock but rather a very thick, ribbed solid wood paneling. I followed expert advice as far as acoustic treatments(room tunes pillows at the ceiling corners,panels on the ceiling to reduce echo near the speakers, a rug with thick under pad to cover about 80% of the wood floor. I have corner bass traps on the speaker wall. The room is overflowing with records and CDs. This phenomenon was present with my previous speakers(NHT 3.3s) as well as my current BBC monitor style speakers. I have tried a variety of experiments to see if I can change the issue. The room has 2 doors but opening doors,windows and/or opening the drapes did not change the situation. Also tried removing the bass traps to no avail. Have run out of theories and experiments at this point, outside of wondering just what materials there may be in the room that absorb this frequency. My equipment has also mostly changed since I first measured a few months back, have mostly Allnic gear at this point. Your theories and suggestions are kindly solicited. Many thanks, LS