You cannot "treat-away" a room mode. Your ONLY option to eliminate the room mode is to either MOVE the subwoofer or change the shape of the room. Bass traps MAY be useful for modes that cause bass level to be too high. Those can be controlled with EQ. But a cancellation (these can be as much as -60 dB or more when measured with precision at exactly the center-frequency of the mode. I would suggest getting rid of bass cancellations from room modes by moving the sub, and use EQ if there are bass peaks that need taming. The rest of the room should be treated to make the rest of the audio range sounds its best. Never use TOO MUCH damping and never use TOO MANY hard reflective surfaces. The best rooms avoid the extremes, avoid placing subs where there are giant suck-outs or peaks, and using room treatments for the mids and highs. Sometimes moving the subwoofer even 6 inches will eliminate a deep room mode cancellation. And also... don't forget to measure what is happening at the listening position. You do not want the main seat to be right on top of a serious room mode. If that DOES happen and you cannot move the main seat, you can place the sub close to the main seat (inches) so the room doesn't influence bass before you hear it (obviously, this isn't practical if you want 6 seats with great bass sound).
By the way, just to be clear, I'm talking about not being able to treat a CANCELLATION (little or no sound at some frequency) with any room treatment or equalizer to bring it up to proper level. If you have a response PEAK in the bass, the best answer is moving the subwoofer to a location without that peak and hopefully, without a deep cancellation also. Do not use measurements with larger range than 1/3 octave or you can miss a deep cancellation.