Polk Reserve R100 on a Denon AVR-X1800H: 8 ohms on the box, 3.6 in the bass
The Reserve R100 is 8 Ω nominal, inside the 4 Ω rating of the AVR-X1800H, but it dips to 3.6 Ω. Fine at normal volume. Go easy on long, loud bass.
Everyday listening is fine. The nominal impedance of the Polk Audio Reserve R100 is inside what the Denon AVR-X1800H is built for. The dip below that rating is a caution, not a thermal overload.
At 3.0 m in a typical room, the stereo pair reaches about 101.5 dB. The second speaker and the room each contribute roughly 3 dB, which is the usual allowance for home listening. That sits 3.5 dB under a 105 dB theatrical peak, so loud film moments are where an amplifier with more voltage starts to matter.
Behind that level: the AVR-X1800H is rated 80 W into 8 Ω, and the Reserve R100 works out to about 86.0 dB for 1 W at 1 m from the sensitivity the maker actually published. Nominal impedance is 8 Ω.
The published minimum impedance is 3.6 Ω.
Do not declare the amplifier unsafe because of the dip, and do not ignore it during long, loud sessions: sustained bass asks for more current than the nominal rating describes. A more powerful amplifier does not erase a dip. It only helps if the current one is running out of voltage and clipping.