Upgrading Sony CDP-X557 ES

I've bought a secondhand Sony CDP-X557ES cd player to replace a Marantz CD63 KI. The Sony is clearly the better player, although it's a much older design. But I feel there is more music in it, and am wondering how to upgrade the player. Should I have a new Trichord- or Tentlabs clock installed? Should I use an external DAC? The Sony only has an optical output. Is that a problem?
Any help appreciated. Below two pics of my new toy.....

The outside....

The inside....