I think the issue is not that tube watts are any different in terms of power; the issue is distortion. Watt-for-watt, tube amplifiers (in general) make less of the distortions that the human ear objects to. The result is that with a transistor amplifier you rapidly run into a harshness that stops you from turning up the volume any higher, in avoidance of pain and discomfort. This does not happen as much with tubes and the result of *that* is that a smaller tube amplfier can often be driven to put out more 'comfort' watts than a much larger transistor amplifier can.
IOW there is a greater percent of 'usable' power in a tube amplifier... generally speaking that is.
IOW there is a greater percent of 'usable' power in a tube amplifier... generally speaking that is.