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.
KLH Model Seven's nominal impedance level is 4 Ohms . 8 Ohms is pretty much the standard impedance amongst stereo speakers and amplifiers so KLH Model Seven is not considered a difficult to drive speaker from Impedance perspective.
KLH Model Seven has a sensitivity of 91 dB. 91 dB of KLH Model Seven is considered a high sensitivity compared to average stereo speaker which makes KLH Model Seven an easy to drive from Sensitivity perspective.
Read our detailed KLH Model Seven Review