Actually the mission on the PTS - [Weekly] The Dread Masters - recommends 156 gear (Basic) for Story Mode and 168 gear (Underworld) for Hard Mode. That tends to suggest that SM drops Arkanian and HM drops Kell Dragon. As a comparison, the currently TFB and S&V weeklies recommend 156 for SM and 162 for HM.
As another poster noted, all the Basic/Elite/Ultimate vendors are gone on the PTS,
Also, not sure if anyone has mentioned this, but to complete the Weekly, you need to either defeat Corruptor Zero in Operation: Dread Fortress or Dread Master Raptus in Operation: Dread Palace.
Yeah you can already see all the boss names in the lockouts tab.