F208s frequency range was not specified. Low-frequency extension: 23Hz (–10dB), 27Hz (–6dB), 31Hz (–3dB). Based on these values, one can assume F208s can safely reproduce 31hz-20kHz. Nominal impedance given as 8 Ohms, but that is not the case.
The impedance ranges between 4 and 7 ohms for most of the audioband, though there are dips to 3.6 ohms at 100Hz and 3.2 or 3.7 ohms at 2.9kHz, depending on whether the HF control switch is set to its maximum or minimum, respectively.
At close to 89dB sensitivity, it seems your Denon should have no problem driving these. However, take a closer look at impedance curve above. Impedance drops to 4 ohms ~30Hz and even below 4 ohms between 70-105 Hz range. This is also the case at higher frequencies between 2-3 kHz range. Denon is rated at 125 W/Ch at 8 Ohms and 165 W/Ch at 6 Ohms. Depending on your listening tastes, Denon appears to be good enough to get the job done.
If you must, you can also use the preout with a separate amp, but it better be something that can provide high current. I have done something similar in the past with my Yamaha DSP A2070. Yamaha had 80W at 8 Ohms and 100W at 6 Ohm. I had Thiel 2 2 speakers and I used PS Audio 200CX amp to drive the those using the preout of Yamaha. Thiels had a similar impedance curve as F208s. But PS audio amp ratings were Output power: >200Wpc into 8 ohms, 400Wpc into 4 ohms, 1000Wpc into 1.5 ohms. Peak current: 64A. Bandwidth: DC to 140kHz, -3dB. Damping factor: >40. Those numbers were way better than Yamaha. I hope you get the idea if you want to take that route.