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

As you all of stated, room treatment is critical. Probably the most important part of your system. You can do more harm than good however, if you don't know what you are doing, or just put some things in the room, thinking it will make an improvement. I had treated my room in a manner that I thought made sense, treating obvious reflection points, etc. However, the cheap products I used were far too thin to trap anything but high frequency and I didn't have near enough bass absorption. I also had too much absorption and not enough diffusion in spots. 

I re-did things with professional advice from GIK and their free consulting/room design service. It is an excellent service and free of charge, but of course they hope you will end up using their products. I did, and the results were 100% better. GIK is excellent and there are many other great products on the market. Spend the money and do it the RIGHT way. 

You can do more harm than good however, if you don’t know what you are doing, or just put some things in the room, thinking it will make an improvement.

More harm than good...depends. Some rooms are really bad to begin with, and almost anything (properly placed) will help. But generally I agree.

And learning REW -- https://www.roomeqwizard.com/ --and buying one $100 Umik gets you on the way.

Read about REW. Watch a couple videos. Learn what you’re trying to address first. Then buy conservatively what you probably need (look locally first) and then add incrementally, measuring all the while. This is how you establish a correlation between what you hear and what the measurements show. Repeat as needed.

Room EQ Wizard is indeed your friend worth getting to know.

My background is in recording studios and I couldn't imagine trying to put a system together without first doing basic room treatment. Between standing waves and reverb time, I would find it impossible to evaluate gear. It's a somewhat trial and error process but there are some basic things any room should have. 

Some rooms may have a fortuitous layout with slanted ceilings or non parallel walls that provide a good foundation. But they can hugely benefit from some well placed absorbing panels to reduce reverb time. Without that there will always be smearing and lack of focus. 

Some people worry about a room being too "dead" but why would you want to hear anything that isn't from the source? The ambience you want to hear is embedded in the recording. The sound of your room only detracts from that. Consider that headphones eliminate the room so there's little reason to want to hear the room. 

mashie ...Consider that headphones eliminate the room so there’s little reason to want to hear the room.

There you go, plug in to your nice headphone amp, one and done. All music.