How do I setup an extra headphone mix from line outputs 3/4 on a Quantum 2 interface?

I recently purchased a new Quantum 2 interface. I'm looking to setup an alternate headphone mix from the Line Outputs on the back (3/4) to my headphone amp. I want to keep the headphone mix on the front as well. On my previous interface, I would setup this via the Digital Mixer; however, routing is limited in this model. I'm using PreSonus Studio One Pro, but it would be nice for a mixer setting that will work in any DAW (i.e. Logic Pro X as well). The headphone amp I am trying to drive with the alternate mix is a ********* HA400 headphone amplifier that only has a single TRS input (not separate L/R input chael inputs). What are my options on doing this? I'm new to Studio One. Thanks!

Here's is my current setup:

• Apple MacBook Pro (Retina, 15-inch, Mid 2015)
• OS X 10.12.6
• PreSonus Studio One Pro 3.5.2.44603
• PreSonus Quantum 2 (Firmware 1.1-11)
• PreSonus Universal Control 2.4 for Mac
• ********* HA400 headphone amplifer