Hi,
I notice that the two frequencies are almost an octave apart, and given the imprecise nature of your measurement, could have been. Thus, while it could be standing waves, it might also be the room resonance and its first harmonic. The fact that your room looks square (nice drawing BTW) doesn't help, as I'm sure you know.
I'm guessing that you're renting, so can't be damaging the walls, and that you probably don't want to spend a huge amount of money on acoustic treatments, which can get very pricey very fast. I would suggest that you spring for the thirty bucks for Everest's 'Master Handbook of Acoustics'. Despite the formidable title, it's written for normal people, and he teaches you all the physics you need to know. I just got mine, and was very impressed by the thoroughness of his coverage. He even goes into the acoustical properties of carpets, drapes, and people in a listening environment! You might also want to do a web search on 'bass traps' and see what you find. There are several DYI projects that might work for you.
You indicated that pulling the sub out of the corner made a huge difference. How far did you pull it? I've seen test data that indicated, at least for speakers, movements as small as six inches can make measurable effects. You might try to reposition the sub in 6"-1 foot increments, and attempt to get it asymmetrical with the sidewalls and see if this helps. I had a similar problem, moved the sub only about a foot and a half straight out from the front wall, and angled it slightly. I also mass loaded the floor corner nearest the sub with a bunch of old wool blankets I happened to have. Room corners horn load bass, actually everything, and floors are as bad as ceiling, but easier to treat. Everest indicates that cotton is also good. It seemed to help and smooth out the midbass bump I was hearing. At the cost (zero) a worthwhile tweak.
