Two more acts announced for Siren:
1) Be Your Own Pet
2) Nine Black Alps

Nine Black Alps are from the UK. Here's their NME page
Be Your Own Pet's NME page is not as populated.

Be Your Own Pet is a Rolling Stone artist to watch
And Splendid says that "Be Your Own Pet are so badass and so young that it wouldn't be a surprise if a group of aging rockers paid decent money to have them silenced, as any band that's this good at this age makes everyone else in the scene look bad."

The Village Voice says that more Siren acts will be announced. They still haven't announced Quintron. Quintron + Nine Black Alps + Be Your Own Pet + everyone else they already announced = 13 acts so far. Probably one more we don't know about.