Another option would be to add a second subwoofer and place it on the sidewall if possible. Multiple subwoofers when properly placed would help to smooth out room modes.
Adjusting the phase will move the peak/null forwards or backward in the room. If all you care about is your seating position, then you could try to tune it so that the bass sounds good whether you are sitting back or leaning forward. The easiest way to do this is to sit in the listening position with a bass test track while a friend or significant other rotates the phase knob. If it is a phase switch with only certain phase shift settings, then you will need to experiment with the speaker position and the phase switch.
Multiple subs is always a better option if you can do it though.
Good luck. Here are some videos you might want to review on the subject.
