Just a thought... Having just recently jumped back into vintage AR (after a 30+ year absence), I've noticed a couple of thing: some AR speakers have pots shot to hell, others simply need a cleaning; AND some grills look fine, while others are badly stained (darkened) over the cut-outs. In my LIMITED experience there is a correlating factor: rock wool. The ARs with fiberglass stuffing have pots in better condition (as has been discussed), and the grills don't have as much (or any) darkening of the cloth over the grill cutouts. Isolated, or is the sulfur leeching out of the rock wool also darkening the cloth?