Well, the Sansui is actually a decent int amp( If it is the one I'm thinking of from the early 80's) but it doesn't have any optical out.
IF you are going from the CD player to DAC then to Sansui, the DAC may help if it is better than the CD player's DAC but still will be plugged in using RCA stereo(analog). I would just hook up things without the DAC and if it sounds good then keep it that way. Or if you can try out a DAC that is returnable and listen to both configurations for a few days and see if you can hear a difference.
Even though JBL 830s are decent speakers, you may not be able to discern a sound difference. One thing you always want to do is spend IMHO half of your budget on speakers since they have the hardest job of turning electrical information into actual sound.
BTW, what model non functioning Nak CD player do you have?
If you have any other questions or concerns please post back on what you have decided.
Be seeing you...