Take a look at this. If done right, I find that copies can sound better:
http://www.genesisloudspeakers.com/whitepaper/White%20Paper%20on%20Black%20CDs%20ver%203.1.pdf
http://www.genesisloudspeakers.com/whitepaper/White%20Paper%20on%20Black%20CDs%20ver%203.1.pdf