What commerical audio system tweaks provide sound quality improvement??

The Best result from a tweak in my system was from Mana Acoustics stands for Naim SBL speakers! 
Dedicated 20amp powerline from the breaker to the System saves you the troubles of unplugging the fridge and all computers and light dimmers while listening. 
Shakti Stones, M'pingo dots, green markers and Black Holes for CDs, i tried them all: they make life Fun! ;-) 

@2channel8: You are definitely on the right track.  I'd add ear wax softener and a bulb syringe to your recommendation; used at regular intervals, maybe 5-6 times a year.  You'll be amazed at the improvement in your auditory prowess...

Vibrapods are excellent at isolation and vibration control and cheap too.

They all change the sound.....its up to the listener to determine if it's better or worse.

hifiJones......Very true....and for many aging enthusiasts on these pages, it would be hearing aids.

Any, and all of the Audio Magic products. It began for me with the fuses. I now use AM’s Pulse Gen ZX’s, Blue Dot Disruptor, ac Power Outlet, Mini-Ref power conditioner, and Speaker Clarifiers. Every time I added one of the Audio Magic products my system improved. I highly recommend the AM PEA devices! I’m using three now, which have literally exploded my system’s soundstage, recreation of air, and greatly improved the natural tonality of all instruments, vocals included. Everything just sounds more real. You can’t go wrong.