i spent 9 months treating every surface of my listening room through trial and error.
the cost was about $350 worth of cloth and thumbtacks, and about $600 worth of Aural T-fusor diffusers.
these changes were the biggest performance boost i ever experienced. this was after living in my purpose built room for 10 years somewhat frustrated i could not get things to sound better. since i did this in 2016 i have been completely satisfied with my room and system.
you can see pictures of my room in my details.
in the context of my room and system it was couch change.