Without knowing about your existing system, let me expand on my 1st response. I play Redbook CDs as AIFF via MAC mini using Pure Music. I owned a DL III with the M2Tech Hiface. Felt it to offer great sound for the $700 I had invested. I upgraded to a Bryston BDA-1 because I wanted more digital inputs. I then upgraded to the Audiophilleo. Both together cost $1600 used.
I feel the Audiophilleo would have been a good upgrade used with the DL III. My total cost then would have been $1100.
Outside the USB interface I question whether the PS Nuwave is that much better DAC than the DL III. Though I have not heard it, based on reports by others here and elsewhere, I believe the W4S II to be a truer upgrade at a cost of ~$1150 given a used DL III now sells for ~$350.
I would suggest you look for a used music fidelity V-link (24/96) or M2tech Hiface (24/192) and try it with your DL III. Cost would be less than $100. The PS DL III has a robust power supply and great analog output stage for a DAC selling for $800 3 years ago.