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.
KEF R5's nominal impedance level is 8 Ohms but can go down to as low as 3.2 ohms. 8 Ohms is pretty much the standard impedance amongst stereo speakers and amplifiers so KEF R5 is not considered a difficult to drive speaker from Impedance perspective.
KEF R5 has a sensitivity of 87 dB. With a sensitivity level of only 8 dB, KEF R5 is a difficult to drive speaker from Sensitivity perspective, so make sure to choose an amplifier with high power if you like listening to your music at high volume levels.
Read our detailed KEF R5 Review