Get your knee pads ready Redbone, because you're gonna go on a crawling adventure! Assuming you have a long enough sub cable, put your subwoofer at the listening position and start playing some music with good bass content or some test tones. Then, get down and start crawling around, listening for where the bass sounds best. When you find this spot, relocate the subwoofer to it. Try running test tones again. If you still get suck-outs at certain frequencies, try moving the sub around some more. An EQ is only useful to destroy peaks, trying to EQ nulls will only make the situation worse.