The following plots show voltage measured at the speaker end of the cable in unloaded/loaded conditions. Since the speaker has separate binding posts for HF and LF, I could also take partially loaded measurements. Fluke 187, which claims to have bandwidth of 100 kHz for AC RMS, was used for measurements. I do not have fancy equipment. Since the amplitude is very small to compared with the amp's full scale, I believe distortion components are negligible and do not contribute to RMS values.
The conclusion I can draw here is that the output impedance of the amplifier is somewhat comparable to that of the speaker. To see which one is changing so much over frequencies, I need to repeat the same measurement with a dummy load whose impedance doesn't change over the frequencies of interest. It would be also of interest to repeat the measurements with a fancy amp, but I currently do not own one.
Also, I tried measuring at the amp and speaker end to see if there is any measurable difference. The speaker cable used here is Canare 4S11 in biwire configuration. The LF side was measured on the speaker side.
Some drop over the cable is visible. I performed the measurements on both L & R channels and saw very similar responses.
Review of the amplifier by Stereophile (Fig. 1) shows that the output amplitudes at 8 ohm and 4 ohm loads are fairly uniformly apart by 0.25 dB up to around 10 kHz. This implies that the output impedance is real (as opposed to complex) and about 0.24 Ohm (which means damping factor of 33 w.r.t. 8 Ohms). If the speaker impedance was also real, I would be able to compute the speaker impedance, but it is usually it is very complex. Interestingly, impedance measurement of 802D3 (Fig. 1) (not 803D3) shows that the speaker impedance bottoms between 100 Hz and 1 kHz and peaks at 3 kHz. That trend is pretty well agreeing with my measurement of the 803D3.
I believe the impedance roughness is the reason why people say it's not easy to drive B&W 800 series. If gain flatness measured at the speaker's binding post is what's important, low impedance has to be maintained over frequencies including the cable. This calls for a lot of feedback, beefy output stage, class D, or some technique I am not aware of. I am interested to know whether B&W designed the speaker assuming an ideal amplifier with no loading effect or used some kind of average amplifier model for tuning.