On comb filtering again:
Comb filtering is an interesting phenomenon as it is in fact being introduced any time you simply move from a mono speaker to stereo speakers. This is a fact that Danny Ritchie dismisses entirely with the term hilarious.
I found your confusion of its application to be very funny (hilarious).
When a speaker has a comb filtering issue, you can't dismiss it as if it is like unto the effects of listening to a stereo pair of speakers. That is not the same thing. For one, much of what is mixed to the left speaker or right speaker in music is not exactly the same. If the signals were the same then it would just be a dual mono feed. It's not. It's different. This is what puts instruments in their own separate space in the sound stage. And, it is why we have layering in the sound stage.
Plus, even if the two speakers do get the same signal, and if you are sitting in the sweat spot, then there is still no comb filtering effect because they are arriving in phase and the image is then placed in center stage.
Your side wall, and often more so, ceiling and floor reflections, can and will cause comb filtering effects also. This can also be exactly the same if listening to a single speaker playing in mono. So room effects have little to nothing to do with using a single or a stereo pair of speakers.
The only way that you get real comb filtering effects from two speakers is if the seating distance is different from one speaker compared to the other. Then at the wavelength of the differential there will be some cancellation. However, there could also be a coupling effect of some of that same energy from room related reflections. It's all of those things added together, on axis response, off axis horizontal, off axis vertical, room reflections, etc, that make up the total power response that we hear.
So for the power response to be accurate the listener must give some careful attention to their room to minimize reflections that have adverse effects. As a loudspeaker designer, I can't control that. That part is on their end. However, as a loudspeaker designer I can control the on and off axis responses of the speakers that I design. And that is the information that the customer is interested in seeing, because that is the part that they have no control over.
Now here is comb filtering at its worst. Ian, please take careful note here. This will help you.
I have never posted this information before or discussed it because there was no value in doing so. At least there was no value in it for me. But this is your speaker and it will allow you to confirm what I am telling you. This is an on axis response of your VP150 center channel. So even by your own argument that there will be comb filtering effects from a stereo pair anyway, does not apply.
Note that the elevated top end is what give many listener that "bright" impression and the listening fatigue. But that is not why I am posting this. I am trying to help you understand something. I had to really make sure I was dead center on the speaker when taking this measurement to avoid the comb filtering effects. So I was an equal distance from each tweeter.
Here is what happened when I moved the microphone over 1". See Green line:
And this was from 1 meter away. And yes, that was a movement of 1 inch.
Now that is comb filtering! Your tweeters that are on each end of the box are now out of phase at just over 8kHz. With this speaker, you can't even get the same sound at each ear.
Now most people are off axis more than 1" when watching a movie. So here is what happens with the off axis response. The Red line is the on axis response again. Then there is 10 degrees off axis (Orange), 20 degrees off axis (Yellow), 30 degrees off axis (Green) and 40 degrees off axis (Blue).
Now that is a pretty rough off axis response.
Can you see the problem now?
Comb filtering is an ugly looking measurement and, like most, our first thought was it must be detrimental to the listening experience.
Ian, it is VERY detrimental to the listening experience. It makes holes in the response.