I was able to pickup the AD500 for a good price so let's see how they compare to the AD700.
In terms of build, both phones appear nearly identical in construction with the exception of the cable and the jack (thicker cable and screw connector on the AD700).
Like many members have reported, the AD500 does sound very similar to the AD700-I would say about 90% identical.
However, there are some differences:
1) AD500 has a more prominent low end
2) AD500 is less bright and detailed-sounds slightly muddy and muffled at times when compared with the AD700
3) Contrary to popular belief, the drivers in both headphones are not the same. (AD500 drivers resemble and sound similar (less boomy) to those found in the Philips HP910 -Perhaps dynamating the back of AD500 drivers may improve mids and highs?
Additionally, the grills of both phones are interchangeable.