Many of the better makes have the capability of paralleling the power transformer, so that it supplies more current at a lower voltage for lower impedance loads. This is one of the few occasions where valve amplifiers can have an advantage. The classical configuration uses an output transformer to reduce the high voltage from the valve to a high current into a speaker. So making them multi impedance is a very low cost added option . The irony being that the greatest compliment for transistor output stages was that they could avoid the distortion of an output transformer!.