Thanks for the reply. There are 2 output devices for the ZX, speakers and SPDIF. The ZX replaced a SB X-Fi Titanium card with a 5.25″ front module, which had separate inputs for L/R, a microphone, and headphones. It also had 2 output devices, the same as the ZX. There is one difference between the cards, though. The X-Fi had a shared jack on the back of the card for speakers or headphones. Without the front module, I would have had to plug and unplug speakers and/or headphones at the back, but the front module eliminated that problem. I never switch modes when audio is playing, as I know that causes problems.
The ZX card has 2 separate jacks on the back of the card for speakers and headphones, so that’s one big difference from the X-Fi card. It does not have a separate output device for headphones, so you’ve identified the issue. The switch between speakers and headphones is done with a Creative SB Control panel, which has tabs for various settings. When I quit and restart DPS, it sets the speaker or headphone level to 100%. I know it does it deliberately, but I forget why. I normally have it set to roughly half volume.