Did Slayer and Anthrax remaster any of their early albums?

Not that I have found, but yes, Megadeth did this by Dave's choice. I think Anthrax and Slayer want to preserve the original quality and essence of thrash metal by keeping their releases the way they are, IMO. They may have a greatest hits album out with re-recorded songs, but I don't see them re-recording whole albums anytime soon. Exodus pulled this off with Let There Be Blood and sounds great! Hope this helps man!