When your SM was new the line in both of these was the same. They were 10mm Kevlar core in a polyester sheath. I doubt there was a better line anywhere in the world. 

If you can find an identical line today it would also likely slip because line manufacturers have taken material out of the core. You may be surprised that a newly made similar construction can be pinched with your fingers to 7mm, while the Amel-supplied 10mm line cannot be compressed at all. Go look at the sheave in those Anderson Line Handlers and imagine 7mm versus 10mm and you will totally understand the issue. Go to any rope sales outlet and pinch the 10mm line, and you'll get it.

The solution is to use a high-quality no-stretch core like Kevlar, Spectra, or Dyneema with a polyester sheath in the 12mm size. Do not use Dyneema sheath as it is too slippery.