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

For the guys who are thinking about digital room control, yes, it is a must. But, you have to use room treatment to resolve the worse issues. If you do not you are going to waste power and headroom,  possibly clip your filters and blow a speaker. Correcting a 6 dB deficit costs you 4 times the power, 10 dB 10 times the power. I have seen plenty of rooms 10 dB off at some frequencies. Without room control you can not make your system perform absolutely symmetrically at all frequencies. It is extremely hard, near impossible to get the best imaging without it. The problem is that no two speakers have exactly the same frequency response. Then you put them in different locations and the variations wider further. Doing this by ear is futile even by millercarbon:-) The best set up techs will use a calibrated mic to test each speaker in the system then retest after adding room treatment. You can resolve the worse issues this way but fine tuning like this, a couple of dB here and there and getting the system symmetrical is impossible to do with room treatment. This is the best use of any room control system. 

I am not a big Dirac fan. I prefer purpose built units like the DEQX, Anthem and Trinnov units. I do not like the Lyngdorf units. They use the most primitive version of TacT's system (Lyngdorf was a share holder of TacT and was responsible for European distribution). Boz never licensed him to use his high power system. Lyngdorf also forces you to use his built in amplifier which has a hard time driving ESLs. I owned the TacT version for a while. So I am speaking from direct experience. 

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.