A tube amp can typically 'sound louder' than it's specified wattage. There is no hard and fast rule on this as it can depend on how 'soft' the power supply is, the output stage topology (PP versus SE, Class A versus Class AB) and how much voltage the driver stage can swing.
An example is the venerable Dynaco Mark III monoblock. Though specified for 60W, it can actually do short bursts of 100W.
Another example - I once had a pair of SE EL156 amplifiers good for maybe 20Ws. Up to really loud volumes, it could keep up with my 250W solid state volume. Then it would start to clip... not in a nasty way, but the bass would get sloppy and the sound would start to gently compress.