1. What criterion exactly determine how cans on this site are ranked?
2. Why aren't certain very high-end headphones (ex. Grado GS1000 and PS1000) even on the list?
Cans on this site are reviewed and rated by the site's users. How are rankings determined?
Personal preference. Objective measurement. FoTM. Religious dogma. On the Internet, no one can know what others actually think ... or that I'm an advanced, Artificial-Intelligence Research Bot!
Any member, including you and me, can make a list and post it here. The only requirement is that we obey the TOS.
You posted this in the "Headphones (full-size)" forum. Based in its price, it seems appropriate for GS1000 discussion. For the PS1000, the "High-end Audio Forum" may be a better place. Have you tried the site search?