Control audio output and volume

Hi everybody,

The last couple of days I tried to find a solution to the following issue and hope that you guys can help me. I want to control my computer sound with a seperate device without the hassle to go into my computer settings.
The following function should be supported:
- switch between different audio output sources (e.g. speakers to headphone)
- change volume
- amp for the headphone jack

I listen to music (50%) and play games (50%) on a stereo headset (Ultrasone pro 900) and speaker (Teufel Motiv 2). The ASUS Xonar Essence One is a little bit out of my league.
Do you have any suggestions what kind a device I could get? Any recommendation is very much appreciated. Thank you in advance.