Bob - Look here http://stereophile.com/solidpoweramps/musical_fidelity_titan_power_amplifier/index5.html
Find distortions vs. power at 1kHz and it shows this u-shaped curve. Everything behaves worse at low levels. For speakers it might be mechanical but for amps and preamps it is probably noise in the system and possibly more nonlinear operation around zero output.
Most amps have the lowest distortion between 10 and 100W (dependent on max power) while distortions at 0.1W might be 10x worse. Distortions also increase with frequency by another 10x.
I don't know about tube amps - will check.
Find distortions vs. power at 1kHz and it shows this u-shaped curve. Everything behaves worse at low levels. For speakers it might be mechanical but for amps and preamps it is probably noise in the system and possibly more nonlinear operation around zero output.
Most amps have the lowest distortion between 10 and 100W (dependent on max power) while distortions at 0.1W might be 10x worse. Distortions also increase with frequency by another 10x.
I don't know about tube amps - will check.