1. Fuses
2. Carbon Fiber outlet covers
3. Cable Elevators
I fully agree with SRB's tweaker's approach. I am assuming speakers, room, equipment are optimally setup, given cost and practical considerations, so where do I go from here? In my setup, I would say:
1. Polarity
2. Fuses
3. Tubes
If I could add another factor, it would be turning off sources of digital noise/hash such as computers and AV equipment