What commerical audio system tweaks provide sound quality improvement??

Make sure your connections are rock solid. I have those awful plastic shrouded speaker connectors on my Marantz and it was always difficult to tighten them down. As the connectors tighten, the plastic shroud and the metal contact seem to move as one but I've always wondered if the plastic tightened first before the metal. I've even read where some people have broken off the plastic since the spades can only be inserted one way and one way only, adding to the problem.

Anyway, I went and had the amp end of the speaker cable swapped out from spade to banana and all I can say is what a difference it made. The connection is so much better that it's the old "I'm hearing things I haven't heard before" reaction to very familiar CDs. All of this from better connections. And yes, I cleaned the spade connectors a few months ago so it's not that they were dirty. :-)

All the best,
Nonoise

Agree to the above. Connections matter. The best tweaks/improvements I have found to make nice improvements, after the room and amp to speaker matching are taken care of, and in my opinion:

--Interconnects. Do not cheap out here.
--Good copper connectors either rhodium or gold plated for your power cables. Speaker cables also.
--Some good power outlets, like the Furutech GTX or the new Furutech NCF.
--At least one of the components needs to be tube based.

Audioquest Binding Post Wrench, best $9.99 I ever spent....for solid, tight connections!

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.