Gert,
The power capacity of dummy loads can be usefully increased by putting them in a container of water (- though not 10x so!). I can pump >100W into my 20W resistors for well over a minute.
If the container is clean you can have audio coffee afterwards.
Makes me think back to a dummy load project in one of the electronic mags from years ago - resistor network inside an oil-filled syrup tin.
