Is there a way to improve the sound?

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

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.

Thank you everyone.  I will follow your advice.  

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.

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."