Kristoffer, I must disagree about the advantages of acidic substrates. It may be true that they discourage bacteria from growing, but they may also discourage amphibians from living. Of course it is difficult to generalize: different salamanders have different tolerances, and sources of sphagnum and oak can vary in acidity. However, it is generally recommended to NOT use either of these substrates. Acidic conditions can be dangerous, as acid disrupts the mechanisms of ionic balance in amphibian skin.
There have been some previous discussions about this with Ed Kowalski - he is a more authoritative source than I am. I can try to look up the old discussions if you want to read them.