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

Are you handy?  I used Owens Corning 703 panels for first reflection points on sidewalls and ceiling.  Wrapped them in fabric, very easy and inexpensive.  Made a huge improvement in soundstage and imaging.  Just be cautious while handling (mask, gloves, long sleeve shirt, etc.) and do out in the garage if you can.  

Whatever you decide, just start with first reflection points and then see how you like it.  It might be all you need.

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...