No load is fine for any solid state amp unless maybe it's monitoring the load and trying to be smart. Resistors will get hot quickly, so over-speccing will be better than under. Make sure it's really in series with the driver, and check your wiring in case you shorted something or accidently wired it parallel. Also look into L-Pads from Parts Express or similar. They're made for controlling the volume between an amp and speaker.
Measuring with an ohm meter will give you DC resistance, but not AC impedance. For this, it will get you close enough, but keep in mind it's common for an 8-ohm driver to have a DCR of 6 ohms.