Uh, sound panels. They're like $50 each. And bass traps too. Using sound panels is, without doubt, the best dollar-for-doaalar improvement you can possibly make. Just at the first reflection points (walls and ceiling) will make a huge difference.
Has there been any inexpensive item that enhanced your system's sound?
For me it was an inexpensive power cord that was laying around for years, a Pangea AC9. I have it connected to my Yamaha A S2000. I've tried the stock cord on the Yamaha and then some of my other cords, (Wireworld, VH Flavor 4, Acronlink/Oyaide custom cord and a MAC). I've had lots of amps and integrated amps over the years, and this is the first time the Pangea contributed to making better sound. It surprised me because I had tossed it aside and forgot about it. Power cord performance is such a crap-shoot and dependent on so many other variables in the audio chain. Any inexpensive item you've come across that enhanced your system's sound?