Can't hear plugin preview audio

I’m running Cubase 4 with 24 channels of D/A- A/D converters with an external analog mixing board. I don’t use the Cubase internal mixer for anything.

When trying to preview audio in plugins, I can’t get any audio. I’ve tried numerous things based on threads of other people having the same problem , but no results. I’ve created a headphone output in the Studio mixer routing assigned to a couple outputs on the D to A converters, but still no audio. Am I missing something in the routing between the plugin preview signal and the headphone output? I find this whole internal mixer and routing overly confusing.

Audio preview is routed to

  1. Main out - if control room is disabled
  2. Monitor 1 Bus (IIRC) - when control room is activated
  3. To the phones channel - only if activated in preferences