Yes, I too assumed that it could be the amp (RX-Z9) leaving a sonic signiture, but tried another (DSP-A1) and even Grumpy brought a JBL amp over and ran it right off the computer all with the same result.
Like I noted, I can get the classic 2235 frequency curve with the mic lying on the floor.
Room does have one wall of large windows and a wall of "French" windows, but these are covered by heavy, light-proof shades. There is also a triangular window near the ceiling. Other walls are typical wall board with some artwork. Furniture is typical overstuffed. Room is approx 28'x16.5' and ceiling slopes from 16' to 8'
BTW, there is no suck-out when you talk in the room.