Why do Harbeth speakers have such a cult following

@mapman 
every harbeth model can convey a sense of vocal clarity and realism because that it the mission of their BBC heritage- to reproduce voices in a clear, uncolored natural presentation.  
Of the several times listening to the 40.1 and 40.2 I felt they were extremely natural sounding even with brighter recordings.  The depth of bass from the larger woofers softens the sound somewhat.  
The brightest , clearest sound to me comes from the SHL5+ of which I own.  I just love the sound of these speakers.  
I have owned the C7ES3 as well and they also have fantastic vocal transparency with a touch more richness from cabinet warmth.
The 30.1 / 2 and the P3ESR to me sound more like the 40.2, clear yet with a warmth and treble roll off that softens the sound just a bit too much for me and my system.  
However all Harbeths can respond quite well to system changes that put the sound in a direction more to your liking including room position,  interconnect cables, speaker cables, tube changes (if you have them) etc.    
To the notion that they are not energetic enough for rock and pop I completely disagree.  System synergy in source and amplification can make them very competitive with speakers that have dynamics as their strength.  

Interesting discussion. I have a couple of comments:

1) I don’t think Harbeth speakers are popular due to effective marketing or just because Stereophile highly rated them. Anyone who has ever relied on someone else’s ear to guide them most likely had been disappointed.
2) they can’t rock. That is false. I listen to a lot of good rock, Yes, Steely Dan, Gregg Allman, Mott the Hoople and many others. They sound realistic and great. They may not add artifacts like ear bleeding treble or artificial bass, but they are extremely realistic and faithful to the recording.
3) There is no “best” speaker in and of itself. You have to take so many their things (room, personal tastes, equipment, budget..) into account

so I think we are trying to answer an unanswerable question...like how many feet is yellow.

i can say that in my room, with my tastes, a Rega vinyl front end, with Exposure electronics make my Harbeth 30.1s sing 

two last things. I have heard it said that Harbeth speakers are what people choose when they are tied to listening to speakers. That is me. And secondly, with the Rega/Exposure/Harbeth set up. I spend a whole lot more time listening to music, and not my system

Good luck on the search. That is half if the fun.

I listen to rock primarily.  Some of the other speakers I've owned did some things better than my Harbeth Super SHL5 Plus.  My Legacy Audio Focus 20/20s and Klipsch KLF-30s conveyed more sense of scale and dynamics because they were larger speakers with more drivers and greater sensitivity.  They just move more air.  If you want to fill a room with sound and really rock out, those are great speakers.  Where they lose out to the Harbeths is on clarity and neutrality.  Instruments and voices tend to get pushed together more and at those higher volumes they can start to lose cohesion and start to hurt my ears.

Now that I'm approaching 60, the kind of rock I listen to is more stuff like Steely Dan, Pink Floyd, Jimi Hendrix, Santana, Steve Miller, etc.  I am not usually listening at high volumes, and I enjoy the nuances of hearing every instrument clearly defined and listening to the words and musicianship more than ever.  Which isn't to say that I haven't played some Iron Maiden, Tool, AC/DC, and the like through the Harbeths.  They can rock and can go loud without the harshness of many other speakers.  Having 4 subwoofers assisting them also helps make up for the smaller size of the Harbeths and gives things more of a "live" feel.  

I get that what sounds harsh to me can be appealing to many. I belong to a local music club.  On a number of occasions we've listened to a system consisting of either a Primaluna integrated or Anthem Integrated and a pair of Klipsch La Scalas.  I love Klipsch speakers, but the La Scalas hurt my ears at higher volumes.  When we cranked them up I couldn't wait for it to be over.  The other guys all loved them.  When I first heard a pair of Harbeths I fell in love with the sound and agree that there are some trade offs, but overall they do so many things right.

Certainly my favorite speakers in this line are the speaker model(s) without ports. I also favor the 2 ways due to the simpler crossovers. 

I agree that the Harbeths do as well on rock as they do other material. No speaker worthwhile does any type of music better than another. Aside from SPLs of course in which case horns are your best choice.