You are very welcome! Let's take some of those issues one at a time. First, with respect to room modes, your couch is not actually at the midpoint of the room. Some bass will escape to the space beyond your room, but the primary room modes are determined by the four walls which enclose the space, and not by the openings to other spaces. So, your couch location is not a principle factor in the mid-bass cancellation, in my opinion. If I'm right, that's a good thing, because that's a harder thing to fix!
FWIW, I think that you will get better overall sound quality, if you move your couch about a foot away from the wall, and put a painting or something absorbent on the wall behind the couch. That will help your mid-range and treble sound quality. Reading Section I of the Guide will help you to understand why that would be helpful, and you will also see some suggestions in that section about running a better Audyssey calibration. The bare wall directly behind your couch is a factor in what you hear, and in how Audyssey performs.
Second, I would experiment with other subwoofer locations before
running Audyssey, and I would experiment with changing the phase on one of the subs after
running Audyssey. It is important to understand that there is a degree of trial-and-error in all of this. You observe a large area of cancellation and try different methods of fixing it, without knowing in advance whether you can find physical locations which will work better. And, you won't know whether you will have localization either, until you try the placement you mentioned. Especially, with more than one sub in a room, nearfield placement of a sub can sometimes work very well.
The same thing applies to making changes in the phase on one of the subs. You just have to experiment to see if it helps. Audyssey sets the distance for each sub separately, but it EQ's the two subs together, based on their combined response. Where you know that you have a large area of cancellation, especially in a portion of the frequency range which is very important to you, you can experiment with a phase adjustment, or a distance adjustment for that matter, to see if it helps. The suggestions I am making are not just random ones, but I still can't tell you which subwoofer locations will work best, or whether a post-Audyssey phase adjustment will be helpful. You still have to experiment to find out.
I think that having better subs with more mid-bass headroom, does represent a good path forward. But, more headroom won't, by itself, fix a null. If the null remains after your Audyssey calibration, the new subwoofers will be able to play all frequencies louder than the current ones could, but that portion of the frequency range, which dips down, will still play softer than the rest of the frequency range played by the subs.
If Audyssey is fixing the problem sufficiently that you aren't aware of any loss of SPL in the mid-bass range (40Hz to 80Hz, in this case), then more capable subwoofers will absolutely give you more of what you want, and they won't have to work as hard to do it. In that case, I think that the two PSA subwoofers would be a great choice, because they are especially strong in the range you care most about. And, I would also expect you to notice an overall improvement in your bass sound quality with better subs.
If however, there is an ongoing issue in the 40Hz to 80Hz range that you are aware of, then some experimentation may be helpful to you, either with your current subwoofers, or
with the new ones. That's really all I was saying.
We all decide for ourselves how much experimentation is enough. Some people really like to tinker with subwoofer placement, and with other things including measurements, and many others just like to put subwoofers where they seem to fit and work fairly well. I think that there is no absolute right or wrong answer to any of this--just the answer that your own sense of curiosity, and/or level of satisfaction, dictates