Yes it can be very dangerous and possibly fatal to mess around inside valve amplifiers if you don’t know what you are doing.
The reason is that the valves need very high DC voltages (up to 500V) to work.
This is way higher than ‘mains’ voltage and it is also DC not AC.
DC is far more dangerous as it doesn’t ‘throw you off’ if touched.
Also, one thing not commonly realised is that these high voltages can be present long after the amp has been switched off and disconnected from the wall. Even after several days of non-use, fatal voltages can remain inside the amplifier.
Why is that?
The reason is the very large capacitors which hold this voltage. Think of them as storage batteries which only slowly discharge.
Valve amp technicians are ultra-careful when working on valve amps because of these dangerous voltages.
It is usually okay to change the valves on your amp of course, as this doesn’t usually involve taking the chassis out.