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

I've experimented with a few different products and the best result so far is a combination of panels, diffusers, bass trap and a lot of time listening and adjusting.
Like @noromance I don't recommend acoustic foam, they have a deadening quality and is more suitable for recording studios. In a listening environment, the frequencies needs to be controlled but remains lively sounding. The first reflection on the side wall next to the speakers are the most critical and makes the biggest difference.
The best I've tried so far is a discontinued diffuser product by Auralex similar to GIK Gridfusor, these diffusers just sound so natural and balanced but they are not the best looking. I'm wondering if anyone has experience with GIK PolyFusor or Auralex T'Fusor on the side walls?

I made (8) 4' x 2' x 2" panels, with fabric, mounting hardware and shipping costs for just over $200 with products from ATS Acoustics.
I mounted 6 of these in my home office and it made a dramatic improvement in the sound of my system.  The other 2 panels got mounted in our family room to help tame some of the echo that was present.

OP:

While I did answer your question truthfully, it is also true that my room, with just the Soffit Traps would still not sound good.

You need a critical mass of room treatment to reap the rewards, so I agree with the statements above that it's a combination of traps, diffusors and absorbers, strategically placed, that are totally worth investing in.

I would not approach this as an experiment of individual tweaks.

Best,

Erik

Anthem ARC Genesis.

Anthem ARC Genesis also