I had a hard time with my sub (SVS SB13) in the corner. I did get some enhanced levels near 20-25 Hz, but ended up with pernicious 20 dB+ nulls near 81 and 110 Hz. I moved the sub around a lot, finally settling on a location between my left front and center speaker and maybe 8" away from the wall. I lost maybe 3 db in the 20-25 Hz range but also reduced the 81Hz null to something <5 dB, and the 110 Hz is behaving better, too.
IMHO, the overall sound is a lot better with that sub out of the corner.
Now I'm planning to go after the 110 Hz null with room treatments. I'm at a bit of a loss since I really don't know what bass traps I want or where to put them. (Kinda hate to just blindly buy some and hope for the best.) Would anybody know what kind of a mode is at 110 Hz in an 18' x 24' room? I believe that if I can figure out the mode shape, that would lead me to correct placement of bass traps, but I have not been succesful trying to figure out mode shape by playing a 110 Hz tone and moving around the room with my SL meter. I assume I want the bass trap where SL is the highest near a wall, no?
Does anybody have an organized procedure for reducing a null like this?