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

A lot of good and varied responses / real experiences regarding room and how significant it is (generally speaking).  It is a complex subject matter and one really important piece to this is context and goals - which always seem to be overlooked in these discussions.

You have a living room and shared space with wife, kids etc.. that presents a completely different set of "goals" and application than a dedicated space with free reign so to speak. 

Obviously the latter gives one the ability to do whatever they want to address the room, which in most, if not all cases is significant. From addressing basic first reflections to a full blown acoustic study and build. 

A shared space in most cases will pose significant limitations on addressing the room, which is totally understandable and one works within whatever limitations - be it shared space or dedicated space.  Both situations require different approaches.  

Can you get a good sound out of a shared space relying solely on spkr setup, sure!

Put that same rig in a room with the proper application of treatment, it will most certainly be a completely different sounding and engaging system > for the better without question.

Room size and shape add another entire set of variables to an already complex subject.  Again, whats your goals, $$, and aesthetic requirements. 

It ranges from plop down sprks and good enough to wanting to get the very most out of ones rig - most of the time, it starts with the room - it's a fundamental piece to your rig hands down - at least IMHO.

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.