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

I totally agree with Mijostyn 3 My absorbers (styrofoam) are behide the Maggies also I use defusers on side walls music is front and center stage awesome sound

ATS is great place to get affordable panels or DIY for most economical.  Knauf Ecose with FSK facing (white poly on one side) is best for taming bass and room nodes.  Not sure ATS offers the FSK version.   I found at insulation supply store.  Built frames of wood from box store, thick cardboard or 1/4 plywood backs that come 2x4'.  Fabric wrapped.   Made 6  2'x4' for about $300.  Very effective.   Corner bass traps prob better but in my modest room would be too intrusive, visually.  I didn't have a first reflection problem.   

NMM thanks for mentioning ASC Isotherm. I have been looking at them for quite some time. seems like a good advance.

I did get to hear a demo room using active bass cancellation as the last NY Audio show, which I’ve always wondered about, but stupidly I forgot to ask them to turn off the devices to hear the effect. I can say that in the room next door the bass leaking through was overwhelming, since it was not cancelling bass at that location, but seemingly adding it.
Different topic but getting the speakers really far out into the room is extremely important, and also listening in mid field can give you a sense of what it it sounds like to get the room further out of the equation.

A rug (wall to wall), toss pillows, throws...

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.