Is there a way to improve the sound?

I am not familiar with the NDS, but very experienced with other Naim digital and I will say the power supply makes a huge difference in the sonics.  If you are not using the XPS power supply, you should definitely audition it.  When I upgraded my CDX2 with the XPS, the improvement was transformative.  The sound was so much more harmonically complete and musically satisfying.  If you are already using the XPS, it's true that the 555 is better, but maybe there are other things to be addressed first.  

Have any of you had a chance to listen to a good DSD system?
https://en.wikipedia.org/wiki/Direct_Stream_Digital
An audio engineer/musician friend of mine swears that it offers the best quality he has ever heard. And he has heard a lot. Unfortunately he lives on the other side of the country so I have not had a chance to hear his system. If I do make the jump I'd probably start here. 
http://primeseat.net/en/
You need a DSD compatible DAC. Apparently Korg is all over this, jumping in with both feet. Several models to choose from. Prime Seat is doing live streaming events/concerts. Sometimes directly from a studio. But this is far from main stream at the moment. This may be my next purchase.

Your interconnect and speaker cables are important ! Kimber, Wire World, and purist . Cables are for life your dog is good for only 14 years

The audio stream website has been very helpful in tweaking my system.  Mike Lavorgna is quite knowledgeable and has online tutorials/articles about how to properly set up a system.  Good luck!!   I also have a very good analogue setup and believe that both vinyl and digital can sound very good depending on the mix/mastering of the piece of music.

I use Tidal too.

The NDS/Uniti employs its own software that allows for pretty comprehensive use of metadata with the WAV files; aside from requiring much more storage capability, there is no disadvantage to using WAV files with the NDS.  The original poster mentions having the NAIM ripper, so presumably he is ripping and storing in WAV unless the means of storing is set up so that the files are converted again to some other format.  

At this price point, the sound should be quite good, so it appears that there is some kind of problem that should be sorted out.  

I like the sound from the NDS (I own and use it), although I actually slightly prefer the sound of the 555 cd player.  The NDS sounds more like current high-end digital sources, the 555 cd player sounds weightier and more old school (more like analogue sources).

Mike Moffet is such a funny guy he would probably laugh at the auto-correct of his mane to Mike misfit. I'm also getting tired of some people saying vinyl is clearly superior to digital when they own $40,000 turntable setups and a $300 CD player.

I listen to turntables as well as digital ,for someone to make a blank statement records are better is total system dependent 
dynamic range far better at least 20-30 db more capable 
the most bits a turntable can produce is roughly 12.5 not even basic cd standard ,noise floor S/N ratio not even close the analog 
sound and resolution good digital has very much so .i play my flac
files directed through my iPad off a dedicated player 
for example a Outstanding buy Schiit audio ,sold direct.
the Multibit Gungnir, $1300 dac, or their top dog 2,300
these multi bit dads are their design 21 bit is max that can be extracted at the moment . Go read their white papers .these 24 bit 
dads,and 32 bit are all in theory. It is what is the end readable amount Mike misfit ex Theta digital engineer using their own design using Analog devices chipsets. Are without question 
unbeatable even at 2-3 x their prices and many awards.
that being said you need quality cables , power cords 
and digital cords .now these little regen units clean the 5v that corrupt the digital signal. Up tone regen plug it in and here the difference.with these days or , the Vacuum tube$5k Lascala ,
or $7k . Lampuzator any of these will player beautiful analog sounding music ,without the hassle !!

There is nothing about ripping and streaming that is simple. Ripping to a lossless format is a small part of the quality equation but there is a host of other factors to consider. Just to prove it to yourself, rip one of your reference cds on a laptop you have lying around. Then download the free version of Fidelizer and run it in purist mode on the same laptop (next time you boot changes made by Fidelizer will be reversed). Re-rip the same cd (unplug the laptop charger for good measure). Compare the two rips. This little exercise should prove to you that ripping and playing from a server is just as tweaky as setting up and playing vinyl and we are just on the cusp of figuring it out.

You can get good sound from digital if you use uncompressed file formats. The problem with WAV files is there is no Metadata (track name etc.). FLAC is uncompressed and maintains the Metadata so I would recommend FLAC. Use Dbpoweramp or Exact Audio Copy for a PC and rip CD's to the hard drive. If you are taking CD's and making MP3 files you will easily hear a difference in quality because MP3's are compressed.

My bet is that your system is ripping MP3's to your hard drive. Ouch! Those are compressed files and they remove lots of sound from the file. It's like the difference between cartoons on cards that you flip so that the characters appear to move (and your brain fills in the gaps) and real life. 

Having said that: Nothing beats perfect vinyl from a great phono, it's the ultimate in an uncompressed file (it's beyond a high definition CD file), but there is a place for Digital. I remember the first Audiophile I ran into as a kid. He showed us his system and had a reel to reel tape player. He recorded his albums and listened to the reel to reel casually, but when it was time for a real listen the albums came out. He explained that by doing that he preserved the integrity of the albums. Additionally, iIt has some other advantages: no pops, clicks or scratches etc. The ability to skip from completely different songs and/or albums easily.

I'm an old fart too and have had to educate myself on this stuff - I still need more work.

BTW on another thread you asked about the Oppo. It's a good player. An incredible value for the money and nearly bullet-proof playing anything you throw at it. Some high end players can be a bit picky. 

Hope this helps. Email me in Audiogon if you need help - I'm not a dealer and I don't have anything to sell. Just trying to help another "old guy."

The NDS works much better with files ripped and stored as WAV files.  I heard a comparison of WAV and FLAC ripped from the same CD and the WAV files actually sounded a little bit better.  NAIM's dedicated ripper, the Uniti, rips to WAV.

Thank you everyone.  I will follow your advice.  

Personally, I would not expect very good sound from CD-level formats (or below). I had my "digital" period but I much prefer vinyl. So perhaps you should reconsider the strategy. If Naim can do DSD or hi-res PCM you are closer to a good solution, I would think.

PC: explorer > click on a ripped file > right click > properties
Mac: Finder > click on a ripped file > right click(2finger click on pad) > get info

You should avoid iTunes for ripping, use free XLD app on mac or someone else can suggest for windows. Always rip losslessly to AIFF or FLAC.
Use a tagging programs like MP3tag on windows or Yate on Mac to makes sure your discs have all the metatags(e.g. artist/album/title/artwork) you need.
Read the archives, this stuff has been explained SO MANY times. Youtube has video examples of people doing it all. Cheers,
Spencer

Thanks for your question Kenny.  I must admit I do not know the answer to it.  I am an older audiophile, and I listen to records and formally CD's though the Bryston.  I thought digital was the future, and would lead closer to the "absolute sound."  The whole system was set up by the dealer.  If you do not mind, how woul I find out?  

What file format do you rip to?
Kenny