That's the linear power range of the amplifier when running a resistive dummy load. Frequency extremes will tend to introduce greater distortion into the signal above and below these limits.
Now, an Onkyo is a good unit (got one myself) but will lack the power to produce much sound below or even near the 20Hz range. A speaker large enough to produce significant volume at this frequency is gonna drink up more power than the Onkyo is rated at. Of course if you're just running a line signal to a powered amp then it doesn't matter so much. Keep in mind most people can't really hear much below about 30 Hz, you feel it instead. Again to do this requires big power.