Also, since it's solid state, it will have a "minimum" recommended impedance. The lower you go, the more effective power the amp puts out, but at the cost of heat. Going below minimum impedance can fry important components.
It's not like a tube amp, where the output transformer "matches" the impedance of the output signal to the impedance of the speaker. Running a lower load in a tube amp strains the output transformer, shortens tube life, and can catastrophically cook the transformer if you're really dumb about it.