What Is So Special About Harbeth?

What is it about the construction or technology that makes these speakers a deal at $2700 on sale versus the $800, 900 or $1,000 that others normally cost? 

That's a straw man, as I don't recall anyone raving about the great value of Harbeths, though they certainly can represent good value on the secondary market.

As to the allure, well, that's well known. A gorgeous midrange, and very accurate tones and timbre, especially with human voices and acoustic instruments.

The smaller models are bass deficient, but for those who don't listen to head-banging material, it is a flaw that can be overlooked, or repaired with a subwoofer.

I have heard a few Harbeths (and Grahams ... same BBC lineage). They are sweet on the right music, as stated here, The competition is tough in that price range, which for me is expensive for a stand mounted speaker with limited bottom end. I have read that they still have to pay a royalty to the BBC for the design, though I don't know that is true. If yes, it adds to the price without direct benefit.

Years ago at RMAF I heard a Bricasti Design DAC and (?amp, not his) power a large three-way pair of Harbeths (ported as I recall) that looked like big monitors.

Classical music is okay and I own some (not enough) and work at it but hearing that setup with classical music made me want to stay for hours. It was stunning sound due in no part to those speakers as well as electronics.  It was one of a very few show moments (over a dozen years) that I never forget....

The Harbeth P3 is one of the best sounding small monitor type speaker that I have owned. IMO, there are many others that don’t come close!!

There are others that compete with Harbeth and are better in some regards but Harbeth has a unique sound.  Very smooth, sparkly, just delicious sounding.  I haven’t heard them at high volumes but low to moderate volumes, they sound really sweet.