Famous last words of the tube amp owner:
"Boy it really sounded great for a second until it stopped working and caught on fire".
It depends on the amp, really, on it's design and what type of tubes it runs.
Removing a set of power tubes affects the load impedance on the primary of the output transformer, and can damage it. It can/will also shorten tube life, as they are operating into that changed load and also end up having a higher plate voltage. When tubes fail because of this reason, a lot of damage can happen.
Some amps can handle it, some can't. It depends on the design of the amp.
It is risky without knowing and an output transformer is an expensive fix. A power soak is a better solution. Good ones aren't cheap, but if you replace a couple of output transformers, a couple sets of tubes, and maybe some other stuff, it will seem cheap in comparison.
Anyway, power tubes come in push/pull pairs in class A/B amps, so if you are going to remove a pair, you should take out two that are connected, so to speak.
In 100W Marshalls, for instance, they are all in a row. The outer two are a pair, and the inner two are a pair.
Find out first.
