For me the first sign of Yngwies tone becoming muddy is on the 7th sign record. Though only at one point, and I'm not sure if it has anything to do with his signal chain, but more to do with how it was recorded. But listen to the fast descending riff at 2:23. It has that muddy tone that you can hear in all of his post 2000 records. Attack is the first album where he went full mud.