What commerical audio system tweaks provide sound quality improvement??

These are common on test equipment, which can be influenced by ambient radio noise because input signals can interact on some settings. For audio, not so much, although at the preamp, the tape out RCA's and amplifier RCA's can be connected. In that case, be damned sure that you don't use shorting caps on the tape out RCA's, which short the signal line to ground.

I suspect that the main advantage is cleanliness, which should not be under-rated.

Acoustic resonators ASI and Shun Mook have a rough idea on how it works but it certainly provides audible improvements. Making your own resonators does the job too.

Alan maher designs his QPI's 0hz, Cbf's and fuse wraps. Amazing improvements for any system, he has a good description on how it is applied and what it does.

Footers to name a few Herbie’s, sort kones, Mooks and Harmonics these are system dependent and based on how the vibrations in your system are acting up.

Clean all of your cable terminations, including power cords with a good cleaner of your choice.

Let’s not forget to clean all non-audio connections and outlets, too. In every room, too. You know, computer, fridge, TV, floor lamps, you name it. Good Housekeeping 101.

@geoffkait  

Make sure you turn off the power at the breaker box before you clean the outlets. PLEASE !