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Daveyf,
We test damphers in laboratory here in USA, In Europe and home. There are many instruments to do testing. Some are simple and we can do this at home. Some are very sophisticated- we do lab tests.
All lab created diamonds are different quality. Different polishing quality too. The worst are G from Europe, The best are A from Japan. There are some in between.
Most of users never understand how stylus reads the grove and stereo grove in particular. In few words when stylus wears it becomes more dull and bigger surface/diameter. It does not go as deep in a groove as it used to and not scanning the vertical groves well. So the more stylus wears- the more mono you hear. Some report this as dullness, some hear distortion, some say the cartridge don't sound fresh and musical anymore.