Your correct about the 50hz dip, but if you look at graph 1, it does not appear to be as severe as 2,3 when looking at the other frequencies around it...

Well, Randy and John, the wavelength at 50Hz is about 23', so if the dip is a result of boundary interference effects this would call for a quarter-wavelength distance of about 6' to some surface to be causing a problem(yes, the distance is from the center of the subwoofer cone, not from any part of the enclosure).

Yeah, that's what is baffling us. We can't FIND that 6' distance, unless Randy is a big guy like me and was standing in front of the subwoofer holding a pool table while watching the meter