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 RP-404C 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-404C II is not considered a difficult to drive speaker from Impedance perspective.
Klipsch RP-404C II has a sensitivity of 94 dB. 94 dB of Klipsch Reference Premiere RP-404C II is considered a high sensitivity compared to average stereo speaker which makes RP-404C II an easy to drive from Sensitivity perspective.
Read our detailed Klipsch Reference Premiere RP-404C II Review