What Newbee adds is quite correct - both frequencies appear to be due to room length mode. The 31 Hz null will dominate as it is one mode with a high signal at each end of your room and a complete null right in the middle (and you probably sit somewhere near the middle). See this - just enter your room dimensions.
Big Book shelves and lots of thick bass absorbers at both ends of room (treat all corners) are your best bet as moving the speakers will NOT help (at least not at 31.5 Hz)
Another option is to run a sub up to 80 Hz and place it up against the side wall between one quarter and one third of the room length. This will strengthen your width axial modal response and may cause a new issue to begin to apppear at 43 Hz - but it will certainly help you hear 31.5 Hz and 63 Hz at your listening position - if you can get the balance right between mains and sub. The idea is to use the mains and sub to balance competing room modes for a more even response. Placing a sub there would be a horrible place to put a speaker but could work a little better particularly for 31.5 Hz mode (which is probably a total suckout).
Big Book shelves and lots of thick bass absorbers at both ends of room (treat all corners) are your best bet as moving the speakers will NOT help (at least not at 31.5 Hz)
Another option is to run a sub up to 80 Hz and place it up against the side wall between one quarter and one third of the room length. This will strengthen your width axial modal response and may cause a new issue to begin to apppear at 43 Hz - but it will certainly help you hear 31.5 Hz and 63 Hz at your listening position - if you can get the balance right between mains and sub. The idea is to use the mains and sub to balance competing room modes for a more even response. Placing a sub there would be a horrible place to put a speaker but could work a little better particularly for 31.5 Hz mode (which is probably a total suckout).