Superuniti...Why no USB?

You could put UPnP server software on your computer, and connect it to the SU over a network. If your computer has a digital output (e.g. Toslink) you could use that. If you want to use USB, you can get a USB to SPDIF converter - search the forum, they've been discussed quite a bit recently.

Will the USB to SPDIF converter allow me to connect to the SU DAC successfully?

Yes, it should do, as the SU has SPDIF inputs (coax and Toslink.) I don't do this myself as I stream to my SU over a network from a NAS drive, but there are plenty of people on the forum who play from a computed to a DAC, so if you need more info on this, I'm sure someone will be able to help.

I will buy a converter fro Amazon and ditch the Rega DAC. Thanks Chris!!

Unless it's purely for convenience with lower quality source material, then your best bet for SQ is a good quality asynchronous USB to s/pdif convertor.  Lots of cheap no-name units available from China.  However, it's probably worth spending more and going for something known. These range from relatively affordable units like the MF V-link and M2Tech Hiface to units like the Halide Bridge and up to the Audiophilleo units.  

However, you may well prefer accessing your computer content via uPnP, and that shouldn't cost you anything...

It may sound strange to you, but not all of the USB to S/PDIF converters are the same regarding their digital output. Please take some time before deciding. There are several threads in the Forum, just search for "USB SPDIF converter".

Edit: Richard Dane mentioned several good ones, the ifi seems to be well regarded as well. 

As Richard says above, UPnP streaming is an option - you just need to put the right server software on your computer (or on a NAS). The SU has an excellent streamer which you would then be taking full advantage of. You do need a robust (wired) network to do this reliably. 

Ok Thanks.

Innocent Bystander. How does that convertor connect? What cable do I need?

It has a USB input (type B socket - the squareish end on standard A-B cables). Coax electrical SPDIF output using RCA ("phono" type) socket. It has its own mains power connector (standard 'iec' type socket), but detects the power on the USB cable to turn itself off and on.

it also has an AES output (XLR type socket) and an IIS output (HDMI type socket)

And it has an LED display showing the sample frequency in use.

I didn't answer the question fully: to use it for USB to spdif you need a usb cable, type A to type B socket, and a cable with RCA plug one end and whatever the SU takes - think it has both RCA and BNC, but check! The spdif cable assembly needs to be 75 ohm, not the 50 ohm variety - it should say in the cable spec.

If it were me, I'd load a upnp server on the computer and stream to the SU. It will very likely sound better, be more convenient (you can use the Naim app) and be cheaper too. 

Having answered the question posed, I don't disagree that if considering options for how to play music on the computer through the SU using a uPNP server and network connection would work - and is less costly than using a USB-SPDIF convertor. Quality of one vs the other will depend on the computer, the rendering software on it, and the SU's streamer E.g a Mac Mini running Audirvana into the DAC has been found by a number of fellow members to offer better sound quality than some Naim streamers, while PC based renderers seem not to have such a good reputation.

....which is all just a way of gently steering the OP towards the idea of putting his music on a NAS 

Good idea! Think of how many months of Spotify subs he could pay for by selling the SU and using an iPhone.

It all depends whether you want quantity or quality......

Why would anyone want to steer in any direction? Up to the OP, just give info.

Because it's simple, cheaper than a Mac, the sound quality is just as good, and you don't have to have the computer on all the time (just the NAS - which are low power, and despite some peoples comments implying the opposite, most NASs are lower noise than most computers).

Fair comment - though I found a Mac Mini made an excellent virtually silent server, with other capabilities. There are others, but i found it highly satisfactory.i haven't elaborated that in this thread.

I agree, they are both very valid routes - and if you already have a Mac mini, then there's no necessity for the NAS (but still useful as backup device).

Agreed - and wherever music is stored backup is vital, but easily overlooked.

...have already suggested spending 10 english pounds each month on 'all you can eat' Spotify convenianty accessed by ones portable mobile device of choice?

 'all you can eat' Spotify is to audio reproduction as an 'all you can eat' buffet is to fine dining!