Can I use my SSP's DAC instead of external DAC with Mac Mini?

Get a mini jack to toslink adaptor. They're cheap and you should have no problem finding one at a place like Radio Shack or a music store. You'll also need a toslink cable if you don't already have one. Then all you need to do is run the toslink cable from your mac to your ssp. Go into the menus on the ssp and configure whatever options you need to for the toslink input. You may have to tell mac os to use the optical jack if you're not getting sound. 

Only you can determine if you like the way it sounds, but it should be OK. It shouldn't cost you any more than $20 to go direct. If you're not happy, then get a dac. Classe processors are one of the better HT preamps for music. 

DBTom2 - no HDMI on the SSP-600, the SSP-800 has them, but mine's an older model. Music is a collection of CDs burned as AAC files and the stuff I bought on iTunes was downloaded as iTunes PLUS format (256).

mb1audio02 - I see that some of the higher-end cable-makers (Wireworld for example) sell a cord with mini-optical to toslink ... I actually haven't bought the mini yet - wanted to make sure I could make it work first. Right now all my music is on my MacBook Pro, but I wanted something cleaner and dedicated that I can drive with an iPad or iPhone remotely. I may decide to re-record the CDs to the Mini rather than defer, though I guess a USB to USB or even thunderbolt xfer would work well. 

There's a lot of negative feedback on the Mac Mini as a computer, but I've not heard any negative on using it as a dedicated music server. PSAudio did a decent "how to" write up here, but it left the DAC questions unanswered.  http://www.psaudio.com/ps_how/how-to-build-a-music-server-2/

Thanks for responses - additional commentary is welcome - this is new ground for me. 

I have a MacMini running Roon with a 2TB external drive containing my library of about 25K songs. About 3K of those are iTunes downloads and the remainder are songs ripped from CDs into 16 bit / 44.1 kHz AIFF (lossless, compressionless) format. Also have a few hundred high-res music tracks. I am delighted with the sound and performance of the MacMini as a music server. I use an LH Labs DAC fed via USB to feed my Rotel preamp. The Mini sits on my Audio Rack next to my DAC.

I must be lucky. The MacMini runs very quiet and my old ears do not perceive the "noise" added to audio by the Mac OS's multiple processes running. There are some who believe the music server/computer should be dedicated to just one purpose: audio and also insist the server/computer should be located in another room/closet. 

I also use the MacMini as a video streaming computer to a 65" TV via HDMI . I use an Apple wireless keyboard and Magic Mouse to control things and stream Netflix, HBO Now, Hulu, PBS, and YouTube as well. So my preference for the Mini also has to do with it doing double duty. There's a lot to be said as well for internet surfing on the big screen with a group of people. Many control options available for iOS devices. Great two channel audio too. (Keep in mind that the video streaming can just as easily be done with an Apple TV, Amazon Fire Stick, Google Chromecast, or Roku device - these are much much cheaper.)

There's a lot to be considered when deciding on a music server and now is a good time to be doing that as many products are available. I love my Roon subscription and the convenience of controlling it remotely from an iPhone/iPad. Roon is also an excellent multi-room system where each endpoint can be grouped for the same audio or controlled separately and each playing different tracks from the library. 

Good luck. 

DBTom2 - Rotel is a sister brand to Classe & B&W -  we're chasing similar goals. I just added the 65" OLED from LG (E6P) so it surfs the net with its own kludgey OS. I plan to connect HDMI from Mini to TV, as a monitor as / when I need monitor for Mini, but don't plan to use it that way much. Mostly the Mini is for remote music playback using Apple devices as a DJ/remote.  

Why not purchase a USB to SPDIF converter. many come up used here on AG.  A original Music Fidelity V-Link (black version) would work for your application. This might best be found on E-Bay.