Nice ad!
I bought my K-500 which I use to the present day in 1994.
Now, I'm interested in the three different versions of the K-500. How do I keep them apart, and what are the differences? I'm curious!
Basically there are three different drivers. The first one, it is believed, had a matte diaphragm and 12 openings at the back of the driver (dampened with white fibre paper). All metal plug and adapter, and somewhat thicker cable. Here's the driver, lots of white under the diaphragm:
The second, and most common version (and in my mind the best, although I haven't experimented much with version No 3), had a matte diaphragm but only 2 + 2 holes at the back of the driver. All metal plug and adapter. Here's a pic:
As you notice on the photo above, there are two types of ridges combined in the first and second version: short ridges at the rim, longer ones at the centre (and I don't want to complicate things, but the K400 had different types of ridges on their drivers as well).
The third version had 2 + 2 holes at the back of the driver, but a clear diaphragm with a simplified ridge pattern: only long, single ones. The cable was the same as on the late K400 and the following K401 and K501: partly black plastic plug with metal adapter and slightly thinner cord. Here's a pic of the relatively rare K500 clear diaphragm driver:
My first advice is to get a fresh pair of pads (see post http://www.head-fi.org/t/562787/akg-k500-love/15#post_8349972 in this thread), it will tighten up the sound and improve bass quality. More importantly, it will show huch much the pads affect the sound.
Also, there is a piece of damping foam in the centre hole at the back of the driver, and this material often falls into pieces. Cut a new one from open cell foam and replace (you open the cup by removing the type sign and undo a screw). This foam affects bass tightness/airiness:
If you feel that the sound is still a bit bright and undefined, do the tape mod (or use blutak) as described in the post mentioned above. It's very easy and leaves room for adjustment of brightness/warmness balance.
I'm sure the later AKGs with varimotion (two-layer) diaphragms may be technically better - K401, 501 601 and 701 in different incarnations - but I have a bunch of different headphones from AKG, Yamaha, Sennheiser, Beyer, Stax etc and for the last six months I listen mostly to the modded K500 in my OTL set-up. Why? Beacause I'm there, right in front of the music.