Well, 37Hz correlates to 15.27 ft (The same formula works both ways: freq == 565/feet; feet == 565/freq.), for whatever that's worth.
If the closet doors are acoustically transparent, then you should get two peaks, one at (width of room), the other at (width of room + depth of closet). That'd be my theory, anyway. In reality, there's probably a bit of "both" going on -- some acoustic transparency, some reflection. Maybe even some absorption.
The theory behind Ethan Winer's bass traps is that it absorbs some of the frequency on the way through, but then a significant amount more on the way back out (after reflecting on the wall behind it) -- which is where the tuning is. I'm almost wondering if there is an optimal place in your room to put something large, heavy, and solid (like at the halfway point?) that might help? Or is that where your speakers are? Maybe you should try putting them off center? Or at least angling them, perhaps? If nothing else, you'll learn something, even if it's an arrangement that you can't live with.
PS You also want to measure the height, there's another mode there. It's usually taken care of by carpeting, but sometimes it's an issue. Heck, you may have just indirectly measured your room, if you find three primary modes.