Dropping the sliders down is a "safety feature", more than likely, to prevent unwanted feedback howl. Inch 'em up until you get something.
Is this, perchance, an intel sound card (or another vendor's) based on an ADI 1981B chipset? If so, you may need to pass some options to it to get it to behave properly. On my old Gateway laptop, the headphone and master outputs were swapped in KMix, so I had to pass:
options snd-intel8x0 ac97_quirk=2
to swap them back. Some reading on your sound hardware might yield an insight that's useful.