What commerical audio system tweaks provide sound quality improvement??

waterzlife...you don't have a socket wrench set?  (that's what I use)

One thing that works for sure, but depending also to yhe refinement of uour system. Fuses such as the Synergistic Black fuses,  look up the resistance 
Index gor ant skeptics the Bottleneck stock Steel fuse has 7 times the resistance 
More then premium Silver gold, fuses or even Copper fuse types. Try them 
Give 75-100 hours comes with a 30 day money back ,Noyhing to loose.
This next is the biggest upgrade Boutique Capacitor upgrades ànd many other 
Possibilities, starting with your Loudspeaks. The vast majority under $15k
Have Xovers that parts are average at best. Putting in Premium parts like 
VH capacitors,Mundorf,Audyn,or Duelund, resistors,the same applies to amplifiers, digital, and preamps.on average maybe 30% of the cost of the item actually goes into build.the rest overhead ,and 50% markup for dealers.I solid Audio for years 
That is why buying a solid piece of used gear, then dedicate money's to upgrade 
The results Dramatic. This is why I have equipment built with the best parts 
And it is still less then at a audio store.

Rather than tweaking I prefer "tuning with tweaks"  but I know what you mean.  

• Selecting components that play well together with the speakers that make music for you

• Good quality cables to allow those components to"sing"
• Clean AC 
• Fine tuning with footers.  Herbie's Audio Lab is a great source for these.  Keep working with footers till it sounds right.

Assuming that you have a nice system which you like, you might want to consider the following. I save my money for the physics - that is, if and only if the physics is solid and uncontroversial. Then I have been known to spend real money.

Those areas are: capacitors (film and foil are the best), resistors (nude Vishays are best), solid connectors (not necessarily expensive, just solid), clean power, clean stylus, and clean records. And the most cost effective tweak of all: little rectangles of sticky jelly-like substance, which can be stuck on things to control resonance, Moongel, for $10. I use the stuff extensively on my air bearing tonearm.

You may notice some high-mark-up items missing from my list. So is everything controversial. That's how I approach the subject: physics first. YMMV

Physics definitely supports vibration control. Particularly for tube components and turntables. Don’t leave them out!

Also, room treatment is very well supported by theory.

Some tweaks, though.. Jeez. I can’t believe people purchase these things. They make some of the stuff the new agers buy look like quantum physics.