Have you ever cleaned your stylus or checked it under magnification? Unless you had a regular routine of effectively cleaning it, the stylus is probably caked with gunk and fuzz. When I got back into vinyl 4 yrs ago, my new cartridge went from sounding good to muffled in just a month or two. Viewing the stylus under a magnifier, all I saw was a big, black amorphous, fuzz-laced blob. Several applications of a dry Mr. Clean Magic Eraser (stroked from back to front, brushing off the abrasive flakes each time) finally cleared off the gunk and enabled me to see the bare stylus again. The sound of course improved markedly.