I just got a pair of AD500's and they were way too large for my mellon and kept sliding around. So I decided to mod them by physically shortening the headband. I unscrewed the tiny screws on the inner part of the left headphone up by the headband, then carefully pulled the pieces apart. I decided to clip about 1 and 1/8th inch from each headband wire to reduce the size.
So you must carefully remove the headband wires from their attachment points. Once you wiggle them free you will see that they are bent 90 degrees at the ends to better hold them in place. I used a razer and scissors to carefully peel back the plastic sleeving from each of the headband wires. Please be sure to be really, really carefull with the headband wire that contains the speaker wire. It is very thin and can be sliced super easy. I actually nicked mine, but did not have an affect on the sound, so I got lucky. I removed about 1 and 1/2 inches of the sleeving from both of the headband wires. So I measured 1 and 1/8inches then I grabbed my heavy duty wire cutters and again, very firmly pressed the wire cutters together then very carefully twisted to snap the headband wire off. BE CAREFUL, it would be easy to jab yourself or the headphone components that are still dangeling.
Once you have cut both headband wires you will need to grab a good set of pliers in order to re-bend the ends so that they can be repositioned where they were. You will need to bend them to about 90 degrees. Just use the part that you snapped off for reference. It may take some reshaping and such to get them just right to fit back in. Then put everything back together (which can be a pain because of the speaker wire) and voila, you have a tighter more comfortable and slightly less ugly AD500 or AD700.
I will try to post some pics tonight to help others.
(EDIT): Actually, since I've got them back together, I'll post some pics of the innards and the process only if someone asks me to. Otherwise here's a pic of the finished product. Sorry there's no before pics.
Sorry about my afterwork skanky wife-beater. :-)
You can see the fat pad mod in this one too.
Here's one of the two pieces of the wire head band that I clipped off.