At a speed of sound of ca 330m/sec, a 40 Hz wave length estimates at ca. 8.3 m, half wave = 4.15 m, pretty close to 14 feet.
I.e., you experience a text book suck out (cancellation of half wave by reflected half wave of opposite amplitude).
As others have stated, move the subs around, asymmetrically.
Good luck!
I.e., you experience a text book suck out (cancellation of half wave by reflected half wave of opposite amplitude).
As others have stated, move the subs around, asymmetrically.
Good luck!