The DAC in your Mac is not bad at all in my opinion. My Dad's 8950 has the DA2 and it sounds pretty good.
I would start by listening to your Luxmans analog outs and compare to the internal DAC. Then possibly demo a DAC or two in your system if possible.
DACs have evolved a lot. My main system has a separate DAC but my second system's internal DAC sounds really good. I actually sold my RME DAC because the internal DAC sounds so good. It was redundant . I think there are definitely some inherent advantages of built in DACs if they are implemented well.
Have to try it and see