"The room can totally wreck, or make, a system"

I live in a concrete apartment and listen in my living room. There’s nothing I can do about the very strong peak  at 90 hz and complete null at 40 hz. The Fabfilter pro-Q3 I could use for the 90 hz in Audirvāna is far from transparent.

Design, including how the space is built, solid, laid out well, etc do the best you can then work around the other uses of the space. This will determine where the speakers are going to be generally located so pretty easy to sort out where the main treatments will be needed the most. The better the design of the space, the easier everything else will be.

If a room is to far compromised then all the treatments one could do might not be enough. I have listened to very high end systems I had to walk away from they sounded so bad yet I have enjoyed simple but well sorted systems what were a joy to listen to.

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40 hz null pretty tough to deal with

90 hz peak, I would look into a trap(s) tuned for 90 hz

Peaks are far worse to listen to than nulls and often easier to fix.

Room treatment is my second most expensive component.