I'm pretty sure the feedback you get is from the pedal not the amp ... That's pretty normal from such a pedal , specially at louder volumes.
You'll need a noise gate pedal to overcome this , & learn how to controle the feedback .

Settings will differ depending on your amp and it's settings , but if your looking for a thick & heavy dist sounding for practicing alone , the old trick is to scoop the mids , crank the bass and the drive.

BTW, Slayer's tone hasn't changed overtime , it's just the tunning and the quality of the recordings !