Heres a good quote from the THD website for their Hot Plate:
''Will it hurt my amp?''
''The THD Hot Plate® will not damage your amplifier. When you play continuously at full output, you cause your tubes to age more quickly than they would at lower volumes. Using a Hot Plate® will maintain the life of your tubes at exactly the same rate, no more or less, as when you play straight through the speaker.
Using a Hot Plate® will also not hurt your transformer any more than playing through a speaker, as long as the impedances are matched (i.e. 8 ohm setting on the amp, 8 ohm speakers and an 8 ohm attenuator). If you are using a well-made amp, then the transformer should last indefinitely, regardless of whether you are driving a speaker or attenuator. If you are using an inferior amp and the transformer blows, it would have done this whether you play through a speaker or an attenuator. The Hot Plate® puts the same load on the transformer as a speaker (which is why it makes such a good dummy load).''
