Don't worry about burning them in. Just listen to them. Over time, the drivers and diaphragm or whatever will loosen up. Most headphones usually get better bass response with a warmer mid range over time.
If you happened to be a compulsive headphone hobbyist and insist on burning them in, I recommend using classical music with some good deep natural bass. I seem to get better results with classical music compared to electronic music. Don't crank them up too loud because you don't want to blow the drivers.
I don't own the AKG 501, but I figured the burning process is more or less the same for most headphones.