I, own both the K500 and K400. A visual inspection shows that the baffles are the same design between the two versions I own. The drivers however, are different with the variation between the material of the diaphragm, and the etched ridges on the outer ring of the diaphragm. These effects certainly cause an effect of the sound which to me translates to better clarity, speed, and neutrality on the K500. The K400 on the other hand sounds warmer and more mids heavy and reduced upper frequency.
K400 driver:
K500 driver:
tdogzthmn was right - there is a difference between the drivers I wasn't observant enough to notice. There are three major differences: the K400 driver has around 60 major ridges, with minor inbetween-ridges on the periphery of the diaphragm (a total of 120 ridges). The surface is shiny. The K500 driver has around 70 major ridges, with minor inbetween-ridges on the inner side (close to the coil) of the diaphragm (a total of 140 ridges). The surface is half-matte.
Some of these observations have been made earlier, but not in detail regarding respective model in the K400, K401 and K500 series. Mr Fitz has certainly made some points on the subject.
The K501 has a totally different kind of ridge, and only 24 of it:
The matte K601 (picture) and very similar K701 have no ridges:
Just like the DT880 of 1980, Lisztian warts and all:
(As an aside, I find the DT880 and K601 & 701 have similar sibilance/high midrange issues, maybe beacuse they lack the controlling ridges and suffer breakups, but the undamped peaks might improve transient response).
I'm not sure my findings are important but I find them very interesting. Acoustical engineers could probably explain the advantages and disadvantages of the K400, K500 and K501 driver designs, like the difference between an Otto or Wanckel engine, but the fascinating thing is that each could have its own virtues, detectable by a sensitive ear listening to good recordings with high quality ancillary equipment.
My favourite is the K500 with the baffle mod described earlier in this thread. What I discovered by a simple pressure test is that the K500 (and to some extent the others in the series) has quite a stiff ridge area, and a softer dome. Many modern designs have much softer and thinner diaphragms (less stiffness and less mass), sometimes with a stiffer dome. They are often more sensitive and bassier, but also less accurate (more distortion). See Tyll's distortion measurements of many so-called upper-class headphones!
My thesis is that the stiff diaphragm gives the K500 a better transient response (quick and with better control) and a tight bass with less distortion, It's not very sensitive and the dynamics are probably somewhat softened. It's a balanced set of compromises that appeal to my musical ear. This beauty sings well:
It would be very interesting if Tyll and Purrin could measure these old AKG models!
Thanks to tdogzthmn for making me think.