1. Play with listener/speaker placement since nulls can cover small spaces. I got rid of the one noticeable suck-out in my current room by moving my seat six inches forwards. Try slightly asymmetric setups - sufficient speaker directivity, side-wall spacing, and toe-in can avoid image shift. I've had very good results from two asymmetric rooms with my Linkwitz Orions, and one failure on stand mounted monitors in my home office with book shelves on one wall where I had to use stereo tone controls as a band-aid.
2. Use multiple sub-woofers.
2. Use multiple sub-woofers.