I agree with gmood1. Also not all usb implementations are created equal and not all dacs are created equal.
I used to use an Emmlabs CDSD transport and DCC2 dac. My PC->USB->Empirical Audio Turbo-2->DCC2 produced the same result. The sound was indistinguishable. So I sold the CDSD because it was less convenient. Part of this was using software that did bit-perfect rips and bit-perfect playback, and bypassing the kmixer, and using a battery power supply, and no ASIO driver, and no resampling. As long as the bit stream from the PC is bit-perfect and the USB interface produces the same or less jitter than your transport, then the PC should sound the same or better than the transport. But it takes all of these things to be in place. You can't play a song through Windows Media Player and a poorly-implemented USB interface and expect great results.
I also owned a Benchmark USB dac at the same time as my CDSD/DCC2. It didn't come close to Turbo-2 + DCC2. I don't know if the problem was the Benchmark USB interface or just the Benchmark dac, but it was nowhere near the same league as Turbo-2 + DCC2. I ended up returning the Benchmark within the 30 day return period. Granted, it was a much cheaper product, but frankly I don't see where all the great reviews are coming from. Very bland sound.
So, you still need a great sounding dac for great sound, and you also need a well-implemented USB interface to let the dac shine, and you need bit-perfect output from the PC.