What you describe is of course physically impossible. Bass travels in waves of different lengths at different frequencies. Its possible to have nulls of no bass at certain frequencies, in fact not only possible but guaranteed. But it is impossible to have nulls of no bass at all frequencies. This is step one in understanding your problem.
Step two is your room. But keep in mind all rooms have this same problem to some extent. Your dimensions 13x21, the width is very close to the wavelength at 80 Hz which is 14 feet. You can play around and find the exact bass frequency at 13 feet it will be a little higher but whatever. Point is to start thinking about it as a problem in math or geometry. 13 and 21, seven is close to a common denominator. The worst room would be where they are all multiples of each other 24x32x8 for example.
You have moved them around a lot. Welcome to the club. With only one or two subs that is all you can do. It never will work but its all you have with only one or two. My guess is you also are following the conventional wisdom and moving them symmetrically. Because someone repeated the line about integrating, or timing, or whatever.
What you want to do with your two is put one in a corner and move the other somewhere very different, like a side wall several feet from a corner. This way you will have two sets of very different bass modes and nulls, and the overall response will be smoother.
You never will by the way get as much bass in the middle of the room as along a wall. Any wall. Any system. Any room. It just ain't happening. You don't even want it to happen! What you want is smooth even bass at wherever you're listening from.
All the advice above is ultimately little more than a here's how to make it a little less crappy type solution. For really good bass nothing with only 2 subs is gonna get you there. For that you need two more. At least.
https://systems.audiogon.com/systems/8367
