Surround working on headphones but not 5.1 speakers

Maybe someone can help me out here.

When playing a project (UE 4.25.4) in the editor (including standalone) on my headphones (connected to Sound Blaster X G6 DAC set to 7.1) everything works beautifully, full surround sound, proper spatialization, etc.

However, the very same thing played on the speakers it’s just stereo. The sound device (Realtek Optical) is set in my Windows 10 to Dolby Digital, and the surround works everywhere else (other games, video playback, etc.).

Any ideas?