I'm sorry to hear that you are not satisfied with our support. Let us find a solution together. For me, it seems that the DAC is not exactly what you are looking for. I think, you are looking for a sound card with an ALSA mixer control. As we do our best to find the best solution for our customers, I have offered you to send your unit back and get it refunded. However, I did not receive an answer from you. Is there anything else I can do?
The DAC works without problems with any application that uses the ALSA sound subsystem. Unfortunately we can't support individual 3rd party applications. As stated, the DAC has no mixer control. Many users use a pre-amplifier after the DAC (that's what we also recommend because it ensures the best possible sound quality), other use software with integrated volume control like Volumio, mpd and others. With these applications the volume can be controlled without problems. We're also working together with the XBMC developers to include optimal support for ALSA sound card in XBMC.
We've also updated our FAQ to answer the questions about the mixer control: http://www.hifiberry.com/products/dac/dac-faq/
I'm looking forward to find a solution that fits your needs.