Transport mods can make a world of difference. The majority of transports have very slow signal edges, which contributes to jitter. As was previously stated, better power delivery to the digital parts makes a huge difference in the jitter. Also, most transports are not well-matched to 75 ohms. As a modder of transports, this is one of the things that I fine tune as well as speeding-up the signal edges. Transports can also have very poor pulse transformers, with either poor low or poor high-frequency response. This will also increase jitter. Also, the signal paths on the circuit boards contribute significantly to jitter. I usually wire these paths externally. The fact that lots of signals share the ground-plane in the circuit board and are run side-by-side with the digital S/PDIF signal adds crosstalk, which again increases jitter.