With my qualifications as a rank amateur(!), I can at least tell you all what I went through in my room. I'm in a rather largish square room, carpeted with a set of built in closets opposite the mixing desk and monitors. For the first few months of me sitting and working there, I thought all was good with my sound. Everything I mixed was ridiculously bass heavy on all other systems apart from my monitors. Having no experience, I thought it was the problem of my monitors, so I just mixed with the bass lower in the mix. Except now I could actually not hear that bass at all in my own mixes through my monitors.
I predominantly record vocals for my YouTube channels, and noticed that any word I spoke beginning with the letter "w" would resonate in the most bizarre way in the room. I then played some tones through my monitors, ranging from 50hz to 300hz. As they were playing I moved my head from mixing position to the left and right, and then went to various places in the room to see what was happening. I realised that from my mixing position, just moving my head slightly to the left and right I could go from loud to almost no sound while the test tones were playing. It just so happened I was sitting in one of those nodes where the bass was cancelling out.
After this I built myself 4 large (100cm x 60cm) bass traps from rockwool. I placed 4 of them around my mixing position. The difference now is night and day. Although the cancellation of bass isn't 100% resolved, I would say it's about 90% resolved. Measuring the actual decibel level of the tones around my room, I can see that there is far less interference now. Only yesterday I finished building another 4 traps, which will be placed on the top edges of this room to try and get that remaining 10% if possible. My biggest problem is now the flutter echo, which is going to be hard to tame as the room is not dedicated to mixing, therefore any option of putting panels of foam all over it will not be met with enthusiasm from my wife.