It definitely sounds like IMD distortion is what you and Amir are describing as this buzzing sound. I guess I just don't listen as loud as some people because I've never heard it in the LS50 or Q150, I also think these smaller speakers should be used with a high pass in place during listening tests. Playing small speakers loud and full range isn't indicative of how they are used in practice most of the time so I don't think it's fair to rate them badly because of that.
Also just a random comment about the preference score. At one time, I compared the Q100, Q150 and LS50 in my room and I preferred them in that order. The Q100 aren't bad speakers but I can't imagine many people preferring them over the LS50, so I'm also starting to question the preference ratings.
FWIW, my listening is always at a low volume. When listening the the Q150 I had a "what the heck" moment with the distortion sound. I only mentioned it because Amir noted it. It could just be that there are resonant peaks in frequencies that have excess distortion from the original music recording.
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On a related topic, one of the many things I'm struggling to get my mind around with the measurements of speakers is that frequency response is the dominant factor in user preference, and the related off axis characteristics. My subjective experience is that there are other elements besides frequency response, and I have whiled away many hours EQ'ing speakers, with mixed results.
My current area of ruminating on the subject of what I prefer in listening speakers is that some amount of cabinet resonance is helpful, especially for vintage/classic rock albums. Modern speakers that are made with very dense cabinets and shaped to minimize any diffraction sound incorrect to me, they sound "uncanny." (think of recordings like Back In Black)
Many people champion the quality of speakers "disappearing" but I find this is not appropriate for some music, as it leads to a poorly localized image, that kind of "floats" around.
Paradoxically, speaker cabinet resonance helps localize the speaker in the room, and this actually stabilizes the image as you move around.
Cabinet resonance should show up in a spinorama measurement, but I have no intuition as to what it would look like. I suppose it should cause deviations in the shape of the curves.
Almost all commercial recordings are produced on a pair of point source stereo monitors, and historically, these were resonant boxes. Any imaging involved in the recording is built around this architecture.