This is very helpful. My thinking is to tackle the bass first and then apply dampening/diffusion as needed or as a secondary step.
I'm renting so not able to make any structural or permanent acoustic mods.
What corner speaker are you using which is getting you down into sub 40hz range? I'll have a pair of HS8s and about purchase a pair of Yamaha PA mains for band rehearsals. Would either of these work for this purpose or should I use a sealed subwoofer?
I never really liked basement studios so when I built my house, I went up rather than down. Living out in the middle of nowhere meant I didn't have to have a lot of mass so I could build with low end energy escaping (cows next door never seemed to mind). Also was fortunate that my issues both measured and actual were not in the sub range.
I was using a Tannor 15 inch sub with a couple pairs of KRK and struggled with getting clarity in bass above 80 as the enclosed chart will show. Also if you get my user name, sub bass wasn't my thing. Even still, 125 was a real pain to deal with and why I have a full ceiling cloud. I used a JBL 305mkii to measure initially and played with the sub a bit. Because of the porting in the 305, it has a resonate bass peak that made things confusing a bit at first. I jotted down a couple numbers with the Tannoy and quickly moved on as sub 40 wasn't my issue. Of note, actual was pretty close to this chart (got my numbers somewhere but it's late).
The handy thing with the spreadsheet is that it gives you a baseline of what your challenges are likely going to be and John's also lets you know how much bass trapping your going to need. From there you can decide what is possible given your particular circumstances and budget. Lots of other people have approaches that are different. For many years I had some treatment which was better than none but far less than ideal. I spent a lot of time with headphones and listening to mixes outside the control room.
One other thing to note, you'll notice the modes are relative, as in 40 & 80. This is typical. If you address the lower, the higher often tends to take care of itself to some extent.