"In addition, while the speaker's impedance magnitude drops to around 4.1 ohms in the lower mids and the high treble (fig.1), it is generally an easy load for an amplifier to drive. However, a combination of 5 ohms magnitude and 45 degrees capacitive phase angle at 90Hz will mean that the best amplifier would be one rated into 4 ohms."
"Sensitivity: 89dB/2,83V/m. Frequency response: 45Hz-20kHz"
Although getting close to 4 Ohms, the 818 is rated to 4 ohms so it is supposed to be fine. Also given the sensitivity you should be fine there too. Depending how far away you sit you may not be able to get to reference but if you keep it below -5 you should still be low distortion. If you find you are getting distortion or clipping at the volumes you listen at then it's time to look into an amp. I use an Emotiva across my front three which really only makes a difference at high volumes which I do once in a blue moon. Its nice to know its there, the system has breathing room, and the occasional at reference fun time is still clean, but in reality it's rarely used. Glad I only splurged on the emo and not a Classe or something.