I don't have any experience with the Quad players but according to the owner's manual the CDP-S player does not have any digital inputs but the CDP-2 player has both optical and coax digital inputs.
If you have the CDP-S you won't be able to use it between the Pure I-20 and your preamp, you would plug the analog outputs of the I-20 into line inputs on your preamp.
If you have the CDP-2, though, you could send either the optical or coax output of the I-20 to one of the digital inputs of the CDP-2 and then connect one of the analog outputs of the CDP-2 to your preamp. That way you would be bypassing the built-in DAC of the I-20 and using the one in the CDP-2 instead.
With the CDP-2, whether the optical or the coax digital output of the I-20 sounds better as a digital source, and whether the I-20 DAC or the Quad DAC sounds better, is just a matter of listening.
Good luck.