What Is So Special About Harbeth?

I have auditioned a few iterations of the 30 series and most of the 40s and can second the points made by @orfeo_monteverdi above (and many others). Harbeths are polite and, when you adjust to their presentation, you get musical cohesiveness -- as many have said: it sounds natural, real.

The frequency extension is not gut-wrenching on either end, but the critical mid-range is well done on both the 30 & 40 series. Not surprisingly, the 40s offer more weight / volume and amplitude than the 30s and can fill a medium-sized room quite well. I too found that electronic music came out too polite, probably because I expect harsh / dirty sounds with this music -- that you don't get with the big Harbeths... (that said, Tupak's voice was very clear!)

I too found that electronic music came out too polite, probably because I expect harsh / dirty sounds with this music -- that you don’t get with the big Harbeths... (that said, Tupak’s voice was very clear!)

The SHL5+ sounds more open / less polite than M30 versions with electronic and rock or dynamic music.

choice of amplification also determines whether they will sound slow, polite or boring.  

an explosive preamp and amplifier can get them moving on a dime- example pass labs class A, vinnie rossie MOS FETS, Cary Audio solid state power amps.    

This is a well-known video relevant to the present discussion.

https://www.youtube.com/watch?v=Y3WpRY-EtX8

I think they sound awfully good, but, as has been mentioned, are rather overpriced for what they are. They do that magical midrange thing so many BBC monitors are known to do.  If that meets your needs, and you don't mind the relatively narrow restrictions they put on your choice of amplifier, and you have the cash, they'll be hard to beat.

That's not meant to sound smarmy.  As far as I can tell, getting one thing extremely right is basically what Harbeth set out to do, and I'd say they did it.  That's worthy of respect.  It just isn't worth what they're charging for it, to me.