It is a well known fact that Vintage Hard rubber sounds great.  But why?  The reason is that vintage hard rubber is more pure than most modern versions and that it lacks the accelerants and additives of the modern hard rubber.  The key to knowing the purity of a hard rubber mouthpiece is the color of the material when sanded; the lighter the more pure.  When you sand a Theo Wanne mouthpiece, for example, it sands as light as a vintage hard rubber mouthpiece indicating the purest, and highest quality hard rubber available today.  Other modern hard rubber mouthpieces sand much darker.  See the comparison photo below.