In the interests of safety, and to acknowledge that this work should not be carried out lightly, here is a useful article on providing an additional earth for hi-fi systems:
http://www.acoustica.org.uk/t/earth.html
I should say that my house uses RCDs, I used wiring with the correct colour coding and safety earth plates and I connected to the main earth connection from the consumer unit without removing any current earth connections to the plumbing. The article does also confirm my findings, which is that sound quality with a good additional earth is improved. Please be careful out there.