I'm starting to think this is really simple :
- the "BBC dip" (slight recession around 3 KHz) is popular because it takes the edge off many recordings and takes out a lot of sibilance
- many speakers implement the "BBC dip" either by design or by accident, including the M3 / M40 / M50 line. The M2 / M22 / M60 / M80 line does not have that dip
- we can argue about woofers & tweeters being the problem until the cows come home -- the "sibilance band" seems to be around the 3Khz range, which is where both woofer and tweeter are active
- any speaker with a bump up in its response in that frequency range is going to tend to emphasize ssssibilance a bit... although we need to be careful because apparently microphone placement in the anechoic chamber can produce an apparent bump in that range
- of all the user controls the audio industry should be giving us, the ability to tailor the 3khz range seems to be one of the most important... (after a BFD of course
) but I have yet to see such a thing. Tweeter controls which give us a step up or down at the crossover frequency have been around for years but I imagine they disappeared because they were controlling the wrong thing
- only a guess, but I bet we will find that a "bright room" is brightest in the same frequency range, ie that a speaker with a bit of a dip in the 3khz range would sound a lot better in an overly bright room. This is probably why the dip is so common in speakers today...
Just a thought.
Oh, and if you want to throw in another variable and REALLY get a headache, have a quick read of the following link. Better yet, do a search on "BBC Dip" and read everything you find... this is still a hotly debated topic after 30 years.