Newb question...using an amp/dac purely as a dac.

If the bass is significantly lower then it very well could be the amp, but from what you are describing the DAC section could also have issues.  The amp section of many devices typically is the culprit as it handles higher voltage if nothing else from the output.  Do you use it via USB or RCA connection?  If you are using it via USB try the RCA side by connecting a stereo to RCA, Y-Splitter to an iPod and run that into the RCA connectors on the styleaudio.  If the problem goes away then you know that it is related to the USB.  If not, it is something in common with both signal paths.

Another thing to try would be to listen to music with a different player.  I know several players that do not copasetically mesh with some devices.  Perhaps you switched players within the time frame you noticed the change?  If not, did you change any settings like bit rate or resolution within your player?

Just looking at the device page the signal seems to travel both through the DAC then into the amp section.  I do not see a way to separate the signal paths easily.

Suggestions:

1.)  Try another player such as Foobar2000

2.)  Try another device such as an iPod, or if you are using an iPod try the USB connection on a PC

3.)  Try another pair of headphones

If you are hearing issues, it is hard to say where the problem is without testing the circuit.

If the unit doesn't have a line output, then it would be difficult to tap the signal, level it off and allow access to it.  It can be done, but it might be more trouble then it is worth if it involves modifying the circuitry.  I wold get the unit checked out for problems with the circuit first and then decide from there.