Most studios that do serious work in digital use a master clock so jitter is pretty negligible. Still, jitter is only part of the equation. The output stage certainly makes a large difference as I'm sure RobHolt and BBV will attest to after hearing the difference between the stock DEQ and the one with a different output stage (actually, not really any output stage). The PSU and algorithms used in the DAC will also make a difference. To a lesser degree, so with the actual components (caps, op-amps, diodes) used inside the thing.

Why do cheap CD players / DACs sound good? Because they are! Do more expensive ones sound better? Most of the time yes. But it is marginal in the grand scheme of things.