Will the 3.5 line in jack allow me to use it as a DAC for my game sound, through the USB and let me use the microphone attached to the headphones at the same time? Will the DAC process the line in for my online games efficiently?
Sigurd3.5mm line in bypasses the DAC and and uses the amp only. If you mean you want to use Dolby Headphone from your sound card to the amp, I'd ditch the sound card and use the U100 for headphone output and Razer's Surround software to get virtual 7.1 positioning with headphones. Then I'd just connect the microphone as usual to your onboard audio and select that as your sound input device and the U100 as your sound output device.