When listening to my HD600 on my old Marantz amplifier I get this cracking sometimes in the left ear.
Could my headphones be broken or is it the amp?
How can I check it?
I suppose there may be a chance it's the cans, but I would first suspect it's the old Marantz. Try listening with the mono button pushed in, just to see if the noise stops.
Last year I bought a late 70's Marantz 2226B and it sometimes had a slight crackling noise in one channel. I sent it off for repair and it's sounded great since then.