unfortunately thats a walkway. id rather avoid that area. but I could figure something out.
I think there's a good chance you can improve your bass response, depending on how far you wanna go. It won't happen overnight and takes some homework and commitment tho. Room modes are frustrating, as you've discovered. I got a MiniDSP 2x4 HD just for eq'ing my subwoofers and it helped a ton. Plus having multiple subs helps too.
My next step is room treatments too, but I've been dragging my feet a little on it. I've done a little bit of research and it's a whole new rabbit hole, and looks like a pretty steep learning curve. There's a lot to it, to do it right. So, so many posts out there of folks who over do it, completely deaden the room and hate the results. You need some of those reflections. I don't know enough to trust myself to get it right tho so I will be contacting a pro, at least to get me pointed in the right direction so I'm not shooting in the dark.