Polk Reserve R100 on a Denon AVR-X1800H: 8 ohms on the box, 3.6 in the bass

The dip falls 0.4 ohms short — and that is inside the rating’s own margin.

Both of these devices are better than their labels suggest. The X1800H’s specification block says 4–16 Ω, not the 6–16 Ω usually assumed at this price. The R100 says 8 Ω nominal on the front of the sheet — and 3.6 Ω minimum a few lines below. The calculation divides one by the other, gets 0.90, and returns impedance marginal: connect it and listen to it, but know where the limit sits.

Why this is not a hazard, and why the rule used to say it was. An amplifier’s «4–16 Ω» is a statement about nominal impedance — the averaged figure printed on a speaker’s label — not about the lowest point of the impedance curve. Every passive speaker dips below its nominal value somewhere, usually in a narrow band near the bass resonance or the crossover, and it holds that dip for fractions of a second as the music passes through. A rating that had to cover the worst point of every curve would read «2 Ω» on nearly every receiver sold. That is why pairs like this one have worked for decades while looking wrong on paper. This page used to compare the dip against the rating as a hard threshold and call anything below it a thermal overload; run across the whole catalogue, that threshold raised an alarm on 48 pairs out of 210, most of them ordinary. The verdict is now graded by ratio: below 0.85 the dip is deeper than the rating’s own margin and the warning stands; between 0.85 and 1.0, as here, it does not.

What «no reserve» actually means. 80 W into 86 dB at 3 m give 95.5 dB, 9.5 dB below the 105 dB reference peak. The receiver is not short of watts — Polk’s own power window starts at 30 W. It is short of margin at the very top of the dial, and what ends the evening there is not a failure but the protection circuit muting the receiver. What takes it there is sustained loud bass — an organ pedal note, a synth line held under a film scene — the signal that keeps the load near 3.6 Ω long enough for the shared seven-channel heatsink to notice. Normal listening never gets close.

Polk tells you where it stands. The same datasheet describes the speaker as «compatible with 8, 6 and 4 ohm outputs». A manufacturer that lists three amplifier taps and prints a 3.6 Ω minimum is telling you to plan for a 4 Ω load — which is exactly what the X1800H is rated for. Where a dip does still change the verdict: the KEF LS50 Meta drops to 3.2 Ω, and on a receiver rated only to 6 Ω that ratio is 0.53 — half the rating, not a tenth below it. The calculator still warns there, and it should.

Level and cable. 14 AWG over 15 ft costs 0.9 % of the voltage and leaves a damping factor of 64; 16 AWG would be enough for this run and this impedance. For calibration: the Triangle Borea BR03 on the same receiver at the same 3 m reads optimal at 99.5 dB — a 4.2 Ω minimum instead of 3.6 Ω and 4 dB more sensitivity. That is what having reserve looks like. It is a description of the difference, not a reason to send the R100 back.