My favorite tweak has to be my custom Daruma type ball bearing isolation footers. These made a huge improvement the soundstage height, width and depth when placed under my DVD player. The bass and treble
frequency extension and resolution were also improved. This type of isolation was also built into my loudspeakers by the manufacturer and the same improvements in bass and treble extension and resolution are present.
Other tweaks I use include after market AC outlets, slave inductors in parallel with the power amp and front end as well as tiptoes,ERS paper,maple butcher blocks,HVAC rubber/cork vibration isolators,Caig ProGold,Nordost ECO III and Windex.
Scotty