I use KDE SC 4.6.4 and have a Asus Xonar STX. The master volume slider in kmix in the range 0% to about 50% corresponds to a zero setting in alsamixer and it also sounds this way. So the real (aka alsamixer) range is 50% to 100% in kmix.
Furthermore, the Mic Boost which is available in alsamixer is possible to be included in kmix.

Any idea?

Cheers

Do you have Pulseaudio enabled on your system?

bcooksley wrote:Do you have Pulseaudio enabled on your system?

No.

If you test the other volume sliders KMix has, do any of them mirror the one ALSA mixer uses?

Basically, it's just the Master slider. The other use the full range, not linearly, though. A value of 50 at the Mic or Aux slider in alsamixer corresponds to approximately 70 in KMix.

Shall I open a bugreport?

Please try to reproduce under a new user first. KMix usually represents ALSA directly.

Same behaviour on a completely new created user account.

I recently switched the sound card (but new test account created after this). Everything works as expected except KMix.

This certainly does seem to be a bug. Please file a bug report at bugs.kde.org.

Bug report