I wish I had some way to hear this, and SACDs. Boulez is one of my favorite Mahler conductors. I rank this M6 as one of my favorites, and I just wish the hammers were more impactful. If they were, it would no doubt be my own top pick.
I think Boulez's Mahler is stronger than we give it credit for. It just feels like most of his recordings are one step below ideal. If the M2 had more organ, it would be one of my top picks. If the M6 had louder hammers, it would be my top pick. If the M8 had better tams, it would be my top pick. If the M9 had a slower finale, it would be my top pick. Et cetera.
Those are just my personal thoughts on those recordings. That doesn't mean they're objectively great.