Jtinn certainly has a point, but I think the greater reason for this comparison was the clipping characteristics of tube versus solid state amps in the earlier days of solid state design. When presented with power demands beyond the ability of the amplifier to deliver, tube equipment has a "natural" tendency to clip more gracefully with far less audibly objectionable distortion. Earlier solid state equipment tended to clip dramatically, noticeably and with a great amount of odd-order harmonic distortion. Instead of shredding one's ears, most tube equipment would just compress the sound level on those peak demands, and the distortion generated was a less jarring even-order harmonic distortion - much less objectionable. Accordingly, to play at the same volume level, one required a FAR higher power in a solid state amp to avoid that objectionable solid state clipping characteristic.
