tube Watts vs solid state Watts

This discussion is similar to the high torque versus low torque engines producing the same horsepower discussion.

HP is HP and IT is what determines the max speed that a car can go.

There is the term, however, of 'usefull' power (HP), which could be used as 'usefull watts' for amplifiers.

Neither. The point is that power (watts or HP)is equal to volts (potential or torque) X amps (current or rpm). The high torque engine produces the HP at lower rpm, thus, and this is debatable, at a more useable range.
Amplifiers (tube or SS)can be designed to be either 'high torque' or 'low torque' for the same power. This analogy can only go so far, so lets not push it.
Salut, Bob p.