I can compare the HP 1000 to some of the Grados I own.
I have a vintage RS-1 (one of the very first pairs, with the strange, almost unheard typesetting) and while its drivers are in very good condition and not pink in color, I think it does have the revered, more neutral vintage "John" (Joseph) Grado sound. And I also bought a full aluminum Magnum v4 from Rhydon (who was trying to follow the steps of Joseph Grado for a more neutral and higher resolution Grado). I got those two thinking they would offer a good challenge for my head time to my HP 1000 but they really don't; the HP 1000 still has the upper hand in terms of resolution and neutrality, and if it wasn't only about that, the HP 1000 drivers also have something special about them that is harder to pinpoint, difficult to describe how it translates into the sound.
The HP 1000 dynamic driver --made in Japan by a headphone and microphone transducer firm (named Primo) and using some of Joe's specification for at least the voice-coil copper quality and diaphragm compliance (to tighten the bass and adjust overall balance, according to what he said to Stereophile in 1991)-- from what I could read and see about it, come from an age where improving fidelity wasn't yet about:
-enlarging the diameter (Sennheiser HD800, Sony MDR-MA900)
-differential diaphragm thickness (Sennheiser's duofol and AKG's varimotion)
-dual material diaphragm and fancy rigid materials (JVC using wooden and Fostex microfiber domes, Sony bacterial-formed tissues succeeded by liquid crystal polymer film, or Koss titanium plated, Focal, multiplying the layers of the same material, JVC nanotubes, etc. diaphragms)
-increasing the power of the magnet (Sony Qualia 010, Beyerdynamic T1, Fostex TH900)
The DH-40 uses regular mylar diaphragms, but it has a "black piston" part on its back (that Joseph called a "rear pressure equalizer" in an Audio [or Stereophile I don't remember which one of the two] magazine interview, also in 1991) which is bonded to the the back of the diaphragm (next to the diaphragm), which modulates its behavior. This carefully tuned mechanically damping piston (vibrating along with the diaphragm) acts like, for example this analogy --a bed placed under a convulsing epileptic person--, to even-out / slow down / absorb / diffuse (I don't know exactly what happens there, but I made a hypothetical explanatory diagram about it) anything erratic or over-excited the diaphragm could/would have produced if the driver wasn't incorporating that black piston on its back.
I think this is the element that makes HP 1000 provoke certain reactions (http://www.head-fi.org/t/638900/oh-god-i-just-found-my-grado-hp-1s and http://www.head-fi.org/t/277545/grado-hp1000-just-a-musical-instrument), or make it special to the heart of us HP 1000 lovers. Compared to regular Grados the HP 1000 is objectively more neutral (other Grados and even Magnums are sometimes considered bright and relatively bass humped) and detailed (which I think can be explained by how clean it appears on cumulative spectral decay graphs compared to other Grados and Magnums). Personally (subjectively) I find it better at handling higher volumes (better behaved, the balance is preserved, the highs remain tamed) and less fatiguing to listen to.
So in conclusion if you are to pick a Grado for your collection, I recommend you the HP 1000 over a vintage RS-1 or a Magnum v4-5. Not only the HP 1000 has better resolution and performs better on graph, their drivers have something (black piston) that makes them structurally, functionally and audibly different, not only from all other Grados/Magnums, but also from most (if not all) of the other dynamic headphones (drivers-wise).
Edited by devouringone3 - 4/25/13 at 4:52pm