In my agenda, tweaks are divided into three categories:
1. Those that have a scientific background that explain how they work.
2. Those that might interfere with the sound, even if there is no current scientific explanation on how they work.
3. Those that have no way to change sound, just because they do cannot possibly affect any physical attribute of the playback system, so they just cannot work.
I willingly test (and use) tweaks from the first category, I am more sceptic about those in the second category (although I don't refuse to give them a try) and I stay away from the third.
Examples for each kind:
1. Various footers, spikes and resonance control devices. Also, wire material and braiding in cables, or surface treatments enhancing the transparency of CDs.
2. The QRT products: not sure how they work, but they do (they did change the sound of my system, although not for the best). Also, claiming that they have an effect on the sound is reasonable, considering they are plugged in parallel with the electronics.
3. Peter Belt and other such "breaktroughs" (BTW, is there anyone still using Cardas wooden pucks or Shun Mook ebony blackgammon thyingies in their system?). Also, devices using Schumann resonators to reproduce the "fundamental" 7.83Hz "Earth breath".
Just my $0,02, of course!