How to fix a lack of bass Null

1. Google room mode calculator. Enter your room dimensions
2. It isn't just front to back or side to side reflections.
There are models which include 2 wall, as above, or 3 or 4 wall reflections. each diminish in importance as wall reflections are added.
3. Sub crawl isn't a bad idea. A little tough sometimes, especially if you have some HUGE sub.
4. Multiple subs. Located asymetrically, they compliment each others peak / null to produce flatter response over a larger area.
5. Experiment.
6. Is your sub out of phase? This can produce this 'suck out' effect you note.

Start changing stuff until you get a result. Even if you KNOW you can't leave it like that, just move things around.
Experiment with speaker placement. READ UP ON THIS.

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).

We haven't actually established it's a null yet. Download some test tones in the 50 Hz area you mentioned, copy it to a CD, not mp3. Play that and walk around the room. If the null is as big as you think, you should notice variations in volume, without a meter. If the volume is consistent throughout the room, you got other problems.

Acoustic treatment in that frequency range is not trivial. However, I once had a hallway closet beside the living room that sucked 90Hz if it was closed. Sounds weird, I know, but I had to open that closet every time.

Thanks everybody great response so far, Ngjockey the room I am using is going to be a master bedroom one day and I built a ensuite of it, I will try leaving the door open never have!With regards to bass there is more up the front and down the back, I have contacted a person who as got all the answers like a Rives Audio, but these blocks are in Australia, I spent all morning taking photo's and drawing a mud map with as much detail as I could give them, I will post what the result are. There is know doubt that it is frustrating, you dont know where to start sometimes as it was only that I was thinking of getting a new cd player that I stumble on a thread that made me think what if it is the room, how many people sell there gear because of the room?

I had a serious bass null at 55-65Hz and experimented with two GIK 244 panels, moving them around in different configurations near the sub until I was able to significantly reduce the null (from -10db to only -2db). Placement ended up being two panels next to one another, forming a 4'x4' mini wall, right next to the subwoofer (within 8" inches) where there was no wall (it is placed near a single wall so open on three sides). Apparently this cut off the specific freq. waves at just the correct spot. It was quite a relief to be able to do this so quickly and cheaply. So it can be done.