allthumbs, I checked out their web page and they use speaker coils to load the amp. This is a good article on how to build a load that gives a dynamic impedance similar to a speaker.
http://www.duncanamps.com/technical/dummyload.html
I have a 10 ohm 100 watt resistor and a 25 watt 7.5 ohm'r on my desk right now. I might just start with a simple resistive load.