There are two main factors that set how easy or difficult a speaker is to drive, these are Speaker Sensitivity and Impedance. A speaker rated lower Ohms requires more current than a higher Ohm speaker. Similarly, a speaker with lower sensitivity requires more power to reach the same volume as a speaker with higher sensitivity.
Klipsch Reference Premiere RP 500SA II's nominal impedance level is 8 Ohms . 8 Ohms is pretty much the standard impedance amongst stereo speakers and amplifiers so Klipsch Reference Premiere RP 500SA II is not considered a difficult to drive speaker from Impedance perspective.
Klipsch Reference Premiere RP 500SA II has a sensitivity of 93 dB. 93 dB of Klipsch Reference Premiere RP 500SA II is considered a high sensitivity compared to average stereo speaker which makes Reference Premiere RP 500SA II an easy to drive from Sensitivity perspective.
Read our detailed Klipsch Reference Premiere RP 500SA II Review