Well, nulls may be unavoidable, but especially for the bass, measure your sub at 3 locations. Also, check out the room simulator in the link below and attempt to keep both your sub and listening locations out of the first few nodes. The best techniques are to use placement, bass traps and DSP together. If you can't do that then do what you can. The bass traps help the DSP work better by de energizing the room modes, like damping a bell.
Lastly, GIK acoustics soffit traps can really help smooth out bass responses.
Yes, DSP helps a great deal with 2 channel. No noticeable delay.