And remember that power amp tubes (valves) should not just be replaced. You'll need to have the amp re-biased for the new tubes. Failure to do so may cause damage to the amp. This is not a do it yourself thing, take it to your tech for re-biasing and re-tubing. Pre-amp tubes can just be replaced, they don't need any kind of adjustment (biasing).
Check out my band:Havoc Din