Set fixed audio volume on external USB sound card

I have an external USB sound card to hook up my PC (Fedora 20) to my stereo amplifier:

http://ift.tt/1ByyI6R

Now, this normally works fine, however, from time to time, I have problems with the volume settings. In particular, when playing videos and music with VLC, the volume settings tend to get messed up. For example, in Pulse Audio Volume Control, I set the master volume to 100%, but then, in VLC, the volume setting is below 100%. If I then increase the volume in VLC to 100%, the master actually goes above 100%, which is not very desirable since it will result in distorted audio. The same thing also happens when playing Youtube videos in Firefox. This is rather annoying since it not only reduces the audio quality, it also leads to unpredictive behavior as one volume setting on the amp can produce a vast range of actual volumes on my speakers.

So, how can I completely disable the volume setting on the PC part, fixing it at 100% and control the actual volume only on the amplifier? In other words, I would like to behave the USB sound card exactly as a CD player, tuner, or other audio source, with a fixed level output.