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.
MartinLogan 15i's nominal impedance level is 5 Ohms . 8 Ohms is pretty much the standard impedance amongst stereo speakers and amplifiers so MartinLogan Motion 15i is not considered a difficult to drive speaker from Impedance perspective.
MartinLogan 15i has a sensitivity of 92 dB. 92 dB of MartinLogan Motion 15i is considered a high sensitivity compared to average stereo speaker which makes Motion 15i an easy to drive from Sensitivity perspective.
Read our detailed MartinLogan Motion 15i Review