dude_500, what computer and OS are you running, and what sound card provides the S/PDIF output? If it's Windows, are you running ASIO4ALL? Is there special control software for that sound card that has a "volume control" that digitally attenuates the S/PDIF audio stream? Since these files are recorded at 0dBFS, regardless of input, both files should produce about 1.5V-1.6V rms at the DAC's output (that is, if there is no mangling of the audio data such as digital attenuation of the volume). You should set all software volume controls to maximum.
What software are you using to play these files? Make sure that all effects plugins are disabled (such as equalizers, bass boost, crossfeed, whatever).
Your S/PDIF measurement shows that there is more bass than the midrange, which is opposite of your original report. This is still incorrect, but I suspect it's not a problem with the γ1, but rather a software settings issue. There is nothing in the γ1 that could cause this kind of bass anomaly, whether it's a boost or loss. The only exception is if you try to drive headphones with impedance less than 300 ohms directly without an amp, in which case the γ1's output coupling capacitors would form a high-pass filter and cause bass rolloff. But that shouldn't be the case here, since you're doing the measurements with nothing connected to the DAC output.
