You will need resistors capable of handling the power produced by the amplifier. A good rule of thumb is that the resistor power rating should be at least 2 times the power produced by the amplifier.
In the case of higher power amplifiers, wire wound resistors may be required and using them must be done with caution. Some wire wound resistors change resistance with frequency (even in the audio band) and others may produce a stray impedance that may alter the results of the test.
The WH-50 series of resistors have been used successfully (search for them on the web) and also ceramic resistors made by Vishay-Dale have worked well.