I like Ezmerelda's list of low cost tweaks (mainly involving your time and energy), but recommend a wool felt surround for tweeters (again if the design benefits from it). My addition will be leveling the CD player/transport (and turntables of course) as mentioned in another thread and checking it/them from time to time to adjust the level as needed.
Top Ten Tweaks
To share info, what are your top ten tweaks. Here are my current ones with one to five asterisks to show effectiveness in my setup.
Dedicated mains ring *****
Deoxit on earth and mains (check earth bonds) **
Tighten connection mains to consumer unit *
Unplug video, tv, computer if in listening room **
RDC cones under speakers *
RDC cones under cd *
wattgate plug on powercords *
custom rack with 11cm thick ply shelves ***
RDC cones under rack, with ply support under *
russ andrews 'sniffer' *
Dedicated mains ring *****
Deoxit on earth and mains (check earth bonds) **
Tighten connection mains to consumer unit *
Unplug video, tv, computer if in listening room **
RDC cones under speakers *
RDC cones under cd *
wattgate plug on powercords *
custom rack with 11cm thick ply shelves ***
RDC cones under rack, with ply support under *
russ andrews 'sniffer' *