The set up here shows a standard USB sound card that produces an optical signal. One reason you might want to do this is to avoid any kind of analog noise that might come from the inner from the Surface’s digital audio converter. Not that is a poor quality its just not the same quality you would get processing the digital signal outside of the Surface with a dedicated DAC.
So with this simple set up i took the optical out and converted it with an external Fiio D3 DAC. While the Fiio sounds amazing, it still might not be as serious as an audiophile might want, for instance push it to much higher end DAC, then out to a high end amplifier. What i did in this set up is run the analoge out from the Fiio DAC to some KRK self powered studio monitors.
The two things used in this set up are the Fiio D3 and a Turtle Beach Audio Advantage Micro. They dont make that anymore but they make theAudio Advantage Micro II which is a little nice since there is a leading USB wire so that its not plugged in directly.
Now there is more of a short cut if you want to still want use an external amp by skipping the optical out and converting a pure digital signal directly from USB and amplifying it. You will want to look for a USB Digital Audio Converter. This basically combines the two shown above. You might want to do this if you want use headphones with an external amp.