The K501s are a sadly under-rated headphone. While lean in the lower registers by today's standards, they are stunning in the middle and soundstage. Replacing the stock pads with those from the K701 gives the K501s a very nice low end boost without causing them to become boomy. A "must have" for acoustic music.

Thank you for this hint, I will test this

Hi Fritz,
could you already test it? I am asking because I own a pair of nearly identical AKG K401. As far as I remember the only difference is that the 501 has even better selected drivers of the same kind.