It does not really matter how far apart they are. A lot of people go with fanless cases, which eliminates the possibility of fan noise from the NUC. When going this route, there really is no need to isolate the NUC from the DAC. The main reason to put the NUC in a different room is so that you don’t hear the noise from the fan when it kicks on. If you are not worried about that, then put the NUC where you want it, and get the shortest USB cable that will reach it but it does not have to be exact. As for USB cables, I like the Wireworld USB cables the best. Even when ignoring the possibility that USB cables can affect your system’s sound, the Wireworld USB cables are well built with nice connectors and they are very easy to manipulate because they are flat, thin and very flexible.