04-11-15: AlmargSorry. That sentence was of course completely misstated. I meant to say that there will be essentially no difference between the tube and solid state amps in their variation of output power as a function of load impedance, as long as the amps are operated within their maximum capabilities.
If that output impedance is negligibly small in relation to speaker impedance, corresponding to a high damping factor, there will be essentially no variation of output power as load impedance varies, within the limits of the amp's maximum voltage, current, power, and thermal capabilities.
Best regards,
-- Al