That long hall is the stairway, right? The room width and the hallway are both 22 feet, which is 25Hz fundamental, 50Hz 2nd harmonic. So the hallway could have something to do with it, especially since your ears are parked right at the halfway point, max amplitude of the resonance. See attached picture of modecalc mode prediction.
You could improve your S/N in the midrange by making a fake wall between the side room and your listening area. But you get used to it, it's not a huge deal unless you are really picky about clarity in midrange.
Is the visio all concrete walls or is there other rooms? Finished ceiling? Insulated? Height?
I like the position of the sub in the previous visio, closer to you. The sub should always be closer to you than the main speakers so that you can time align them with the sub delay (phase) control. Out of alignment sub will make the bass sound blurred.
Depending on how low you crossover a good place for the sub might be right behind your chair.
You increase the S/N ratio a lot having it close. The reflections are delayed and much quieter which cleans it up a lot. You can turn it way down in volume so it won't generate as strong modes in the room, and it is closer than the mains so you can delay it to align perfectly with the mains. And it is still out of the way. If you cross it above 80Hz then it must be near the speakers because it starts to locate for most people at 100Hz. You may want smaller 8" separate stereo subs at the speakers for >50Hz, and a big sub closeup for <50hz.
See if you can do a FR sweep with that omnimic? With both speakers playing and the sub turned on... as a baseline, or is it bass line?