B&W 606 S3 on an Onkyo TX-NR6100: 100 watts do not cancel a 3.7-ohm dip
The 606 S3 is 8 Ω nominal, inside the 4 Ω rating of the TX-NR6100, but it dips to 3.7 Ω. Fine at normal volume. Go easy on long, loud bass. Published specs.
Everyday listening is fine. The nominal impedance of the Bowers & Wilkins 606 S3 is inside what the Onkyo TX-NR6100 is built for. The dip below that rating is a caution, not a thermal overload.
At 4.0 m in a typical room, the stereo pair reaches about 102.0 dB. The second speaker and the room each contribute roughly 3 dB, which is the usual allowance for home listening. That sits 3.0 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 TX-NR6100 is rated 100 W into 8 Ω, and the 606 S3 works out to about 88.0 dB for 1 W at 1 m from the sensitivity the maker actually published. Nominal impedance is 8 Ω.
The published minimum impedance is 3.7 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.