Originally Posted by vanwil
The only connection between the dac and the computer is via USB. So how come I can adjust the volume from the flash players?
Just in case you might want a general answer before someone with more technical know-how chimes in - that's because volume can be digitally altered,* so basically you are altering the signal as recorded (same thing if you set VLC Media Player beyond 100% as it goes up to 200% to boost too quiet video). You are basically altering the data that will be interpreted by the DAC, whatever cable/digital protocol it goes out through. While EQ basically takes place at the same stage, which is digital, EQ tries to smooth out the response, so it's a compromise. Volume control at the digital stage, AFAIK, limits the "bits" on the recording, instead of applying a potentiometer to the analog signal primarily to match the rest of the equipment (the sensitivity of the headphone/speaker, the power and gain of the amplifier after the preamp, etc) as your listening preference requires.
*Note - do not confuse this with a digital potentiometer, which still affects the analog signal