There are three precautions you need to be aware of with valve amplifiers.

The first one relates to heat. Valves generate and use a lot of heat. It is important to allow sufficient ventilation and airflow around your amplifier. If placing it on a shelf, be aware of the effect of the rising heat on anything on the shelf above. 

It is a bad idea to store your chocolate Easter eggs on the shelf above the amplifier, for example. Chocolate soup is a devil to clean out of the insides of an amp.

There are ventilation holes cut into the bottom of the case to allow air to flow up through the insides. It’s important not to block these by placing the amp directly on carpet, for instance.

The second one is a specific caution. It is dangerous to operate a valve amp with the speakers disconnected. The speakers form a vital part of the circuit and you can overload the output valves and transformers if it’s powered on without them connected. So always make sure the speakers are connected, whether or not you’re actually playing any music through it.

If you need to disconnect the speakers to re-route wires, for example, then turn the amp off first.

The final one is just to turn the amp off when you’re not using it. This will prolong the life of your output valves and save on power – valve amps are quite inefficient and can not be considered “green” electronics by any measure.