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 Q950'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 Q950 is not considered a difficult to drive speaker from Impedance perspective.
KEF Q950 has a sensitivity of 91 dB. 91 dB of KEF Q950 is considered a high sensitivity compared to average stereo speaker which makes Q950 an easy to drive from Sensitivity perspective.
Read our detailed KEF Q950 Review