What's here are some bits of information that may be useful to someone. This is more like my notes rather than advice, and the stuff here could be wrong because I'm always learning, so critically weigh anything that you find with your experience.
Also check out my blog for pieces that are more opinion and less factual: http://guitarmusicgear.blogspot.com/ I also tweet about amps that I'm working on if you're interested in the blow by blow--@TubeAmpGuru. I'd love to hear from you!
Standard precautions apply. Amplifiers work with potentially lethal voltages that remain even when the power is off and the thing unplugged. Discharge the filter caps and follow basic safety precautions. If you don't know the basics, it's not a good idea to play around inside an amp.