Interesting thread, and think this comment by Atmasphere is very significant,
Because some odd orders and in general higher ordered harmonics are present, which the ear uses as loudness cues, the amp will just tend to sound louder at this point, but without breakup that accompanies hard clipping.as I have read many times that the distortion created by the soft, gradual clipping of a tube amp is perceived by the human ear as an increase in loudness, therefore thinking tube amp watts are more powerful.
As far as guitar amps, I have read that there can be a difference in the design of the volume control circuit between tube and solid state. A solid state amp will reach maximum output with it's VC at very close to fully clockwise to avoid hard clipping, while a tube amp will reach maximum with the VC at only 3/4 of the rotation. This allows adjustment for varying amounts of that desired distortion in a tube amp.