If your system only consist of the CD player, speakers, amp and pre-amp and if your CD player has an internal volume control, then take the pre-amp out of the circuit altogether and run the CD player analog output directly to the amp. If you want a pre-amp because you have other sources, then a different pre-amp may be the way to go. Also, I can guarantee that your analog output from the CD player is not up to snuff and a DAC will dramatically improve your CD player/transports sound.
For most CD players, the internal DAC is a weak link 1) because they are not typically as good as the digital circuitry in a stand alone DAC and 2) the power supplies are inadequate for the analog output stage.
I'm not sure if your Sony ES unit has a volume control, but if it does, nix the pre-amp if that is your only source.
enjoy