If you really want to bypass the Nova streaming board and use it as a DAC you would need to use a USB to SPDIF converter. Better to run a UPnP or Roon sever on the Mac and use it over a network connection, the way it was designed to work.
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An xpsdr connected to an ndac would provide all those same benifits to the ndac. The xpsdr connected to the ndac would in this case though be providing those benifits to what some believe to be an arguably better burr brown chip. 
The nd5 xs2 connected to the ndac is only powering its own transport and digital out - that’s the point of using a better external Dac.
The ndx2 also has the same facility to use an external Dac.
The digital streaming board inside the NDX 2 will be greatly upgraded with the XPS DR over the ND5 XS 2 regardless of any DAC or the Holy Grail of chips. Plus the elimination of a noisy transformer.
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I have a Topping D10s and can play from my computer using VLC. Excuse my ignorance, but that is working like a UpNP? I’m stumbling around using it.
I’m not sure that’s exactly right. When you add an external supply to the NDX2, the better supply takes over powering the whole of the NDX2 - the streamer, the dac and the analogue stages.
When you connect a power supply to the nDac, the latter must stay connected to the mains. The power supply therefore doesn’t provide the better supply to all parts of the dac. From memory, the PS only powers the analogue stages. Whether this makes the slightest difference I’ve no idea.
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This same question came up with the cdx2.
Whether it was best to put the xps2 dr on the cdx2.2 or on the ndac for the best sound.
Richard Dane gave the simple answer.
“If you’re running the CDX2 digital to the DAC then put it on the DAC.”
Lots of discussion to be found on that topic which might be helpful.
The transport digital sides of the ndx2 and nd5 xs2 are the same the differences lie in the in the converter and analog circuitry. Oh and of course any sonic downsides to having a a small tv screen 
Looking at images of the two streamers they both use the same transformer. I don’t know if the transformer remains powered in the ndx2 to drive the screen or any other components. Has anyone put their ear to an ndx2 to check?
Presumably you are using the Topping as a USB to SPDIF converter? If so you are bypassing the UPnP streamer in the Nova completely and using it into the DAC. If it works and you like how it sounds that’s great, but if you run a UPnP server on the Mac you don’t need the Topping. If you wanted to use the Mac to access streaming services that are not supported by the Nova I could understand that, but for playing locally stored music it seems unnecessary. Either run a UPnP server on the Mac and let the Nova discover it over your network, or stick the music on a USB drive and connect that directly to the Nova.
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Perhaps your dealer belongs to a non profit organisation ? 
Perhaps these thoughts might help…
I remember on the old Naim Forum, (about 2012), there was a detailed analysis of CD5XS vs CDX2 vs CD5XS+nDAC vs CDX2+nDAC vs CD5XS+nDAC+PSU vs CDX2+nDAC+PSU.
I think it was a dealer that had made the effort to do a comparative analysis and report findings. At the time, it prompted a lot of positive debate. Maybe this can still be found on the Archive.
Anyway, looking at comments from users, it might be possible to establish a hypothesis, based on a similar proposed hierarchy of sound quality - good to best.
ND5 XS2
ND5 XS2+nDAC
ND5 XS2+nDAC+PSU
NDX2
NDXS+PSU
NDX2+nDAC+PSU
There may be some nuances in the middle there. Any users with actual experience, please do chip in, test, amend or confirm this hypothesis.
However, any agreement on a hierarchy still leaves room for each user to establish for themselves the relative or comparative difference of interest to them and their system choices. (That’s also what I was attempting to say in my earlier post). Go do a demo’ to be sure.
The nDAC is a super product. I for one, would love to see a decent standalone USB DAC in the current Naim product range. It really adds something valuable to system developments
Hope this moves this useful discussion forward.
@Richard.Dane
With respect to the OP, I’m thinking this is definitely thread drift and may be better as a separate discussion. Please take a view - feel free to edit and migrate some or all these related posts to a new thread.
FWIW, there is another thread currently running - “ND5 XS2 and Naim nDAC” - which makes for more interesting reading on this topic.
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No. The onboard NDX 2’s one is slightly bigger, they’re not the same. Anyway I’m out.
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I would but I don’t know anything at all about setting a UPnP up. I have totally befuddled Apple service with how I screw stuff up. I swear I’ve heard “I’ve never see that before” several times. So if I had someone walking me through I would.
How much music do you have stored on the Mac? The simplest option by far is to copy it to a USB drive and put it in one of the Nova USB ports from which its built in UPnP server will find it.
You can run a UPnP server on the Mac, I would recommend Asset. You may find the built in Naim server a little limited in versatility, or not, depending on your needs, in which case a full featured server like Asset would be worth considering.
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I’ve got quite a bit. But since I bought the Atom then Nova I’ve been streaming entirely. Just recently I broke out the old NAS and accessed my music. I dumped a file in the VLC and can play it there. And the artwork shows up, even though I’m playing through the D10 which I totally don’t understand. But yay…
If you’re happy with the sound quality and don’t mind having the Topping in the chain I guess there’s no harm in keeping it. The directly attached USB drive would allow you to get rid of it, and the cables. The limitations are that if you add more music to the USB drive you have to remove it, connect it to a computer, then put it back in the Nova and let it rescan the drive, which can take it a little while.It’s easy to try, just get a USB stick, put some music on it and see how you go.
What is your ‘old NAS’? Most people who run a UPnP server do so on a NAS which also stores their music so this might be another option.
It’s a 1T Buffalo is the make.
I spent a long time at my Dealer’s listening to an ND5 XS2 and an NDX 2The NDX2 was clearly superior. I bought an NDX 2.
Later I added XPS DR to the NDX 2 
Later still added HICAP DR to my Supernait 3
In my opinion one should aim for the best possible front end and upgrade from there.
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I have sort of figured it out…lol. I did manage to drop all my music into the VLC and can now open the player and it’s all there to play. Also, I can play through the D10 or the Nova (or the Mu-so or the TV or the mac) and when I do play through the Nova ( I assume that’s streaming?) then artwork from all my music is displayed which pleases me. I have a lot of music, like 10 days of playing with no repeats…lol…so this tickles me. Plus I have some stuff that’s unavailable on the services. So cool. It’s a bit clunky but hey. So here’s my stupid question: does the Nova need be connected to the internet to play from a stick or the computer? (I said it was stupid…but I don’t mind asking dumb questions. I always say I’d rather look dumb than be dumb.)
No, it just needs to be on your network so you can control it with the app.
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Glad you are up and running. The USB thing would allow you to ditch a few of those cables! You do, as HH says, need a connection to a local wired or wifi network, or even the USB won’t work and the Naim app won’t find the streamer, but you don’t also need an internet connection.
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I think you can probably play from the computer without the Nova being connected to the internet, in theory….
Have you given up on Apple hires and now use Tidal and/or Qobuz?