What do you mean with the bolded part? And what I'm wondering is if you can with the Omni select either each of those 3 options, speakers, headphones or 5.1 surround when headphone is plugged to the headphone jack.
Believe me I've tried every possible config with the ZxR and hoped it would be something I've missed, I've 3-4 times took it out of the box and did some tests only to come to the same conclusions as last time, no matter how I config it I can't get satisfied with the soundstaging, it's too closed in and too diffuze imaging. I'm willing to give maybe Omni a try in hope for that it has a better configured "HRTF"/speaker mode processing.
The bolded part answers your question that you just asked.
The z series and omni do not allow headphone hrtf to be sent through the linE outs. The device automatically choses which surround algorithm to use based on what ports you chose.
Switching to stereo/5.1 enables line out and switching to headphones enables headphone out.
I think your distaste with the imaging may be attributed with the headamp IC itself? As you said the line outs sound good to you no? The only difference in stereo and headphones selected is the port it comes out of and the type of surround. So the hardware must account for the sound difference.
I'd you really want to test that theory out, run some music in asio for both line outs and headphone out. The asio will use its own driver and bypass the z driver and in turn its DSP effects or any "software imaging" you may be hearing.