Using a heftier output transformer as Alan Douglas suggested might be a good idea. I have seen a couple of ferrets with burnt output transformers. Better test speakers (e.g., Wright DeCosta) include an inductor in series with the field coil resistor substitution bank because the inductance of the field coil was used as part of the power supply filtering circuit.
Unfortunately, most (if not all) commercial test speakers of the past used custom transformers.
Here is an article showing how to construct a test speaker using standard components:http://plymouthcolony.net/starcity/radios/pages/unispkr.html