What room treatment product have transformed your room or audio system?

All you need are nice thick acoustic tiles in the right locations. They are dirt cheap and you can get them in many different patterns. I like the 4" thick ones. https://www.thefoamfactory.com/acousticfoam/acousticfoam.html

The right places depend entirely on the type of speakers you are using and the room. For common omnidirectional speakers first reflection points on the front wall behind and between the speakers and on the side walls next to the speakers are the most important places to deaden. For dipoles like Magneplanars just directly behind the speakers is fine. 

Most people only use passive materials, acoustically absorbent, reflective and diffusive one... It is some work to look for the right balance between these three...Passive acoustic treatment is ALWAYS necessary...

But in a SMALL difficult not so  ideal irregular room we must use also more active devices...

I recommend a grid of Helmholtz tubes and pipes, which will activate the room ...When rightly adjusted for your ears/room it will improve the relation speakers/room...Description is in my thread and they cost peanuts ...Easy to make...

We listen to our active room not only to the speakers EVEN in near listening, unbeknown to most...

Acoustic devices are not secondary addition or tweaks, they are more impactful than most upgrade...

Helmholtz is fine and I am on board but can we have a how-to? 

The ones that really worked the most for me are Synergistic Research HFT. I have the full Speaker Kit on both speakers, and I forget what level they call it but several sets in the room. If you look close you can see some of them on my system page.   https://systems.audiogon.com/systems/8367