If you're conecting a SPDIF you have to check the strength of the signal, some DAC's output from 0.5V, 1V , 2V, 2,5V , 5V. Check the maximum rated input for the essence one. Usually the DAC/AMP are made to work with a specific level of input and apply Gain accordingly. so if you're way under the maximum rated input you'll get low volume.
A friend had this problem with NFB-6 because he was using a DAC that has 1V output, so he had to mod the NFB-6 so it would apply a higher gain.