I usually mp3gain to 89db, but that shouldn't make a difference with the
output from your phones. Try another set of cans and see if the crackling
still exists. See if it exists in all music, just to make sure it's not the source.
If your cans are crackling, you've got a dud set. I don't recall reading about
this problem in the forums, but that doesn't mean anything by itself. I'd say
contact Shure and they should take care of you without any issues. Their
support has been reported on here to be very good, and very happy to make
sure that the customer is happy.
If you bought them locally, you could try just returning for a new pair. You
could also return to an online vendor, but if you're going to ship, you might
as well go with the manufactuer.
good luck
-Jeff