I am sure jitter is some of it, but I would also not rule out noise injection via the USB cable. Of course Rega claims the inputs are both isolated and reclocked to correct jitter, but something is obviously going on. Just got the Black Cat Veloce S/PDIF cable and it kicked the sound up another notch even without being burned in yet.
The rig is quite impressive sounding (at least by my standards). Still have stock tubes except for rectifiers in the Woo, and THe S/PDIF cable and PS Audio Soloist Premier need more burn-in. I'm about $3000 poorer, but I'm happy.