[andpgud]

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?

 

[Fearless1]

Check for hair on the driver.

 

[OldSkool]

Can you try a different set of headphones to see if you still get the noise?
 
If no noise with different cans, follow Fearless1's advice and check for a stray hair on the driver.
 

 

[andpgud]

But is there a chance they might be blown?
 

 

[OldSkool]

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.
 
YMMV.