I have done demags on well over 100 cartridges without any problems. It has always worked. I did once accidentally demag a MM cartridge and it completely ruined it, so be careful.
The tell-tale signs that it needs to be done is a loss of high frequencies, soft, flat overall sound with a lack of dynamics. No sparkle. Demaging will usually bring all of this back.
The tell-tale signs that it needs to be done is a loss of high frequencies, soft, flat overall sound with a lack of dynamics. No sparkle. Demaging will usually bring all of this back.