Headphones & speakers on at same time

I’m using a Raspberry Pi3b+ with a X400 V3.0 Amp + DAC. Also a 5" waveshare touch screen.The playback source is selected as iQaudIO Pi-DigiAmp+. When I plug headphones into my DAC they work but the speakers also stay on as well. What do I need to do to just have the headphones work when I plug them into my DAC & have the speakers off?