You are probably confusing the output power rating of the amp with the output impedance of the amp.
The output impedance of a good quality modern audio amp using semiconductor circuits will be MUCH lower than the impedance of any speaker system you connect to it. It may be of the order of 0.01 ohms or less. But that is irrelevant to the poiwer rating of the amp.
The number you are interested in is the power rating, which should specify the nominal impedance of the speaker system (e.g. "100 watts into 4 ohms").
If you connect a speaker system with lower impedance than the power rating specifies, then you will be able to overload the amp, and that could produce audio distortion, thermal shutdown, blown fuses, or more serious damage.