I'm not familiar with your amp and CD player, so I don't know how they're affecting the signal.
One very important and generally well-known issue with the K501s is that they start out with very stiff drivers and need a lot of burn in. I've had a pair for a couple of years and it's my second pair. I bought the first pair before I knew much about better quality headphones. After listening for a few days I felt the 501s sounded congested, noisy, and generally unpleasant and I returned them. I then proceeded to audition about a dozen and a half other phones, learning along the way that I needed to burn them in significantly before judging them. After hearing all the phones within my budget range I bought the 2nd pair of 501s. They just started to open up around 100 hours, and continued to improved for several weeks of steady play after that. In fact, after a few months their sound seemed to become a bit constricted and less extended again, so I subjected them to several 24/7 days of bass test files, 20Hz-20kHz sweeps and a wide range of music, which seemed to help.