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 R-51M's nominal impedance level is 8 Ohms . 8 Ohms is pretty much the standard impedance amongst stereo speakers and amplifiers so Klipsch Reference R-51M is not considered a difficult to drive speaker from Impedance perspective.
Klipsch R-51M has a sensitivity of 93 dB. 93 dB of Klipsch Reference R-51M is considered a high sensitivity compared to average stereo speaker which makes R-51M an easy to drive from Sensitivity perspective.
Read our detailed Klipsch Reference R-51M Review