Using the RZ with NI Complete Audio 6

Hi

This is my first post. Go easy.

I have recently got an NI Komplete Audio 6 interface to use when producing. It is therefore linked up to my monitor speakers I was using when having a mix on the DDJ RZ

I have the RZ going to computer via USB of course, set the output within Rekordbox to the NI interface, and can hear sound from the speakers playing from the RZ and through Rekordbox.

However, the headphones output disappears when using a different sound card.

Surely there is a way around this?