That depends entirely on the speakers. Given the same level adjustment does not necessarily mean that the receiver would provide anywhere close to the same amount of power on two completely different sets of speakers. B&Ws are pretty power hungry and they really need a solid amp behind them in order to sound their best.
You are right, but it also depends on how loud he likes, room dimensions and sitting distance. I sort of took some of those into account based on the stated minimum impedance of those B&W speakers and other facts he provided thus far, along with some assumptions. I then did some quick math (See note), and found that the AVR may not be putting out any where near its limit. He's talking about distortion when "someone talks", not cymbals or bass drums, cannon etc. So even if the vocal part (someone talks) has 12 dB peak, his AVR still won't be at its limit unless he listen at very loud average SPL, like over 85 dB, in a large room, or both.
I would bet its his source media, especially since he said it sounded much better with Spotify App, I mean even Spotify App music wouldn't be of very good quality. It will be good if the OP will give us more feedback on the kind of program material format/specs when he was getting the distortion.
Again, as I said it could well be that his AVR ran of steam, I just don't think we should jump to conclusion at this point. Let's get more info first, sound reasonable?
Note: If he did Audyssey right, at vol 0 he will get up to 85 dB from his main sitting position when playing movie DD or DTS sound tracks. Assuming he sits 4 meter from the speakers, with room gain he probably get 97 dB at 1m (assume only 3 dB of room gain and only 3 dB of gain due to multiple speakers, say just 2). So at vol -10, he would get about 87 dB at 1m. Based on the nominal impedance of the the 685 S2 and sensitivity, the AVR will be outputting 1W per channel. You need 20 dB peak above the average spl he listens to to get to the Denon's rated output.
Those speakers do have impedance dips but again, he's getting distortion when someone talks not when the bass drum hits hard of cymbals crashes hard.
These are of just rough calculations, but it should get us in the ball park. Bottom line, we need to know more about his room dimensions, sitting distance, types/formats of the source media/program material etc. Yes, getting more power is always a good and safe bet, but it is not always an absolute need in every case.
