Has there been any inexpensive item that enhanced your system's sound?

Check the mounting bolts/screws on you woofers and sub-woofers.
Re-tighten if necessary.

asked children to go to their room.

Perhaps the best inexpensive item was many years ago. It ws my first power cord upgrade. My first experience was with the Nordost Shiva. My local dealer called me and said that he had something special he wanted me to hear. So I went to the store.

I ended up buying the Nordost Shiva power cord and using it on what used to be Rotel's flagship CD player, the RCD991AE. 20 bit, balanced ouputs (not fully balanced unfortunately) and adjustable dither. Even had a defeatable digital out. Immediate improvements in several areas. The music had better focus and soundstage. The bass, solid and tight. It also smoothed the top end. Great little investment. Still have the player and the power cord. 

The second cheap enhancement is the "Gutwire Notepad". Placed it over the transport of the Rotel. Also on top of preamp power supplies. I even used them on a second older system consisting of an all Audio Alchemy front end with interesting results. I've had a Notepad in my system since.

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Before having dedicated line installed I changed my outlet to a Hubbell industrial grade receptacle. Results may very. My old one was aged and worn hardly held up an AC cord. But they were clean.

Next I had dedicated line installed. The only sonic improvement I realize was when other appliances aren't in operation sharing the line.

2" maple block with spikes under amps.

4 lb bronze bar on top of disc player with cork fasted to the underside. It can be anything weighty to keep cost down, I just happen to have some raw metal around.

Custom triple footer maple rack for sources and preamp (total cost $120)

Good results can be had by changing preout/in loops to better quality RCA connectors. Even something that won't put a big dent in your wallet should net you an improvement.

Blue Circle thingee line filters.

Mogami wire

Listen late at night and early morning.

Clear out the area between your speakers also give them enough room to play in all other direction. Some room adjustments are free but net big results in terms of sound.

Catoctin Creek Rye and Red Breast Irish Whiskey. Enjoy.