Your system probably has to be of high quality to be able to hear improvements of some tweaks, IMO, but some that have worked and become significant to me:
1. Just introduced to Auric Illuminator kit, a variation of the green pen on CD, it blackens CD edges and treats CD surface-- very worthwhile. Optrix CD treatment smooths CD music presentation too, but the Auric kit is quite a bit better.
2. NOS tubes in pre-amp.
3. Hal-O tube dampers (or others) on pre-amp tubes.
4. Seismic Sinks under pre-amp and amp; or other vibration control methods.
5. Good quality outlets, ie Hubbell, FIM, Acme and others.
6. Dedicated AC system.
7. I too agree w/Swklein tweaks. Clean your contacts.
8. Good quality power cords.
9. ASC (or other) room treatment.
Cheers. Craig.