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.
Polk S55's nominal impedance level is 8 Ohms . 8 Ohms is pretty much the standard impedance amongst stereo speakers and amplifiers so Polk Audio Signature S55 is not considered a difficult to drive speaker from Impedance perspective.
Polk S55 has a sensitivity of 90 dB.
Read our detailed Polk Audio Signature S55 Review