What do you do when a customer brings in a fixed bias amplifier? "Release" it so different brands of tubes can be put to the ground and find your tone. In this case a Peavey Valve King came in for a power tubes replacement. Being a fixed bias amplifier forces you to buy the brand selected by the device manufacturer and still I would not trust it without checking bias.
Usually this type of modding will open a wide range of tonal possibilities. In this case it is basically a buffer-safety resistance in series with a variable one, chasis testing points and 4 matched 6L6 GC.
The original RUDY are replaced with JJs and bias adjusted to a safe level for normal power disipation and of course pure guitar tones.
Here are the 6L6 GCs ready to rock!
Another common situation is the need to take up the rains of some amps in order to deal with output volume, take advantage of it and use it as a volume-boost. It is clear you will limit the power stage punch but is always faster and cleaner than an internal modification.
Taking advantage of the effects loop can be handy to deal with this, the combination of a master volume and a true-bypass is a nice tool to overcome the problem and be able to boost parts of the piece.