Help me kill that last mode!

My control room is around 4,5 / 4,7 / 3.0 meters (I know - bad ratio) - solid walls, square (I know - equally bad). I have installed two super chunks (one at the top right corner, and one at the bottom right corner), 12 broadband absorbers / bass traps where ceiling meets wall (stradling 95 mm rockwool), 8 broad band absorbers on the rear wall and a cloud covering most of the ceiling, suspended about 20 cm from the ceiling. There's no absorbtion on the front wall. I'm actually very happy with the result apart from one last thing: That 70 hZ mode of death that haunts my listening enviroment. The ringing is really bad at that freq. I can't pack anymore absorbtion in there!!!! I know eq'ing is a bad idea. Any hints on how to kill that frequency - I think we're talking a 10 dB peak at least - vey narrow and very nasty ringing. Note: this has been posted in gearslutz as well.

build a panel trap tuned to that frequency and put one in the center of the back and front walls. see Ethan Winer's site on how to build one.