30Hz SBIR null would indeed be around a 19ft path length difference (i.e. the reflected sound travels 19ft further than the direct sound). I have not seen your measurements but chances are at 30Hz this null could more be just a hole in your frequency response where there are no supporting room modes (since the spacing between room modes is quite large down that low, in the 'sparsely populated modal region').
Below I have posted a crude drawing of the room. My thought is to eventually place some bass traps on stands to be able to place into the alcoves when I am listening to music. But, I don't know if it would work best with; 1. The traps straddling the corners in the alcoves (green). 2. The traps placed across the opening of the alcoves (red). or 3. The traps placed flat against the back wall of the alcoves (in front of the closed doors and a few inches out (blue).
Anyone have any thoughts on this, or anybody else here who has had to deal with alcoves? I'm thinking the alcove at the front of the room is probably the most problematic. For one thing, all three doors must stay shut. The alcove at the back leads into a large bedroom, and generally the door is open. I have performed measurements with the door opened and closed. It really doesn't seem to make any difference.
Also (sorry, so many questions), Nyal or Ethan... In Wes Lachot's article, Bass Waves in the Control Room, he mentions avoiding deep bass traps just behind the listening couch. I assume he is NOT talking about membrane traps (like Ethans)? Plus, since the article is about a control room, is the couch referred to, one which would sit close to the back wall, and not necessarily a normal listening position?