Oh what an idiot am I. I did not read the first post correctly and thought the OP was trying to launch HDX without an interface.
Try holding down the down the N-key continuously. from launching until the playback engine diaolog (or error) appears. Try using an apple direct wired keyboard.
To confirm the N-key is working. Best way would be to have the keyboard viewer utility on the screen and see the key being pressed. (Set that up in Systems Preferences>Keyboard>Show Input menu in Menu Bar) and then open "Show Keyboard Viewer" under the keyboard icon now in the top Menu Bar.
More likely the key is working (or you'd have noticed it elsewhere). And something else is corrupted.
Does other audio, say from iTunes play though the Built-In Output?
Does Audio-MIDI Setup open correctly? Delete all aggregate IO devices.
Trash Pro Tools Prefs and restart (Use Peter Gates PT Prefs, try trash everything).
Create a new admin user and try opening Pro Tools from there.
Shutdown, pop out both HDX boards, try again -- be careful not to dammage the *TDM* flat interconnect cables between those HDX cards
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