[nst7]

You can use the TRS outputs that way, but they will still be controlled by the main volume (CR on the volume knob). So whatever you have the volume set to for your monitors (through the Dsub into your amp), the same volume and signal will be sent to the TRS outputs.

You could then boost the volume more (or lower) on the headphone amp.

This is not really an ideal way to do this. The main reason for the TRS mirroring is for someone who only needs to use two outs for stereo monitors to not have to buy a d-sub cable.

You have a couple of other options. One is to run a stereo 1/4 cable from the headphone out jack of the Omni to your headphone amp. You could then adjust the volume output to your headphone amp separately from the main CR output.

The other way is to just use a couple of the other analog outputs (3-4, 5-6, or 7-8) as normal line outputs into your headphone amp. Doing it this way, you can set up aux sends in Protools to go to those outputs.