The limiting factor isn't just wattage - the audibility of clipping comes into play as well.
Tube amplifiers "soft-clip" - that is, they do not generate as much harsh-sounding high-order harmonic content when they clip as solid state amps do.
Perceived loudness is more a function of the average SPL than of the peak SPL, and a tube amp's relatively benign behavior when clipped means that it can be driven farther into clipping before the distortion becomes audible and/or objectionable. So the average SPL with a 20 watt tube amp might be 2 or 3 dB higher than with a 20 watt solid state amp by the time clipping becomes audibly objectionable.
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