DACs That Improve The Sound of the Sony HAP-Z1ES

Some discussions of this here in the past, but mostly ad hoc rather than anything comprehensive.  For those using a DAC that both improves the sound of and works seamlessly with the Sony, I would appreciate your input as to what your are using, and if enough of a positive change to justify the expense. And even though every now and then I think about it, in the end would prefer to not move to computer based solutions, love the simplicity of the Sony.