What DAC can improve REGA Saturn?

Better check into a seriously good digital cable, too, or else you'll be potentially wasting your time with the DAC. On a very good system, the digital cable can have a large effect on how good the DAC sounds. As with any other part to a hi end system, the sky's the limit on such cables. I surely would not skimp on it, or else the end result of the DAC's performance may not be to your liking.
You also should determine if you want a SS DAC or tube DAC, depending on what you want improved from the sound of the Saturn.

Richard, I've used Tara Labs and MIT recently in reviews on Dagogo.com. You can get some sense of difference in manufacturer's products from those reviews. The Magnan Digital Cable was very refined. However, you might find the MIT to give you more excitement with the Saturn as transport.