Those of us running Pro Tools HD have to jump through a lot of hoops on Mac in order to use the HD cards (HD Native, HDX) for default system audio. This is why it's possible to set audio output to Avid/Digidesign in Logic, Cubase, Reaper, Final Cut and others.
I'm encountering this very problem: OS X, Avid HD Native Card, Tascam DSM-7.1 surround controller. I do surround all day long in Pro Tools but have so far been completely incapable of getting WWise to acknowledge my HD Native card.