Hi Regina, I have done some investigation of this over the years. Previously I used mini disc and then laptop to play backing tracks which I stored on my (Windows) PC as mp3s. There is a tiny discernible difference between them and wav formats but it's only really noticeable to the human ear if you wear headphones and listen VERY carefully! In a live performance situation the differences are entirely UNnoticeable, honestly because there's so much background noise and there are also reflections off walls and ceilings, etc...
When using my laptop I had noticed that quality from the headphone socket was, well, rubbish to be honest. 90% of Windows PCs and laptops use the cheapest sound interface built onto their motherboards. They are not good for high quality performance. When I used a laptop I bought an external sound card and played music through it and compared it with the headphone socket. The difference was remarkable, very noticeable and so I always performed with the external sound card (MAYA USB44).
However, for the last 3 years I've used iPad for three reasons:-
StageTraxx is a brilliant player and can be used to display lyrics/chords as well as organize playlists easily. I could not find anything similar for the laptop.
It is much cheaper than a laptop. My first iPad was bought via eBay for £150 and was great, I've since upgraded to an Air 2. The last laptop I had cost £700 and was built to my specification to include a SSD drive containing all my tracks and programs. Trouble with it was I also had to splash out £85 for the external sound card and a similar amount again for a karaoke player - which was the only thing I could find to allow me to setup playlists and play a single track then pause. Most players don't do this, they simply play a track from a list non-stop which as you'll appreciate is no good for live performance when you need to stop between songs
The headphone output on the iPad is far superior to my laptop one. Apple hardware is superb and has never failed me in 100s of gigs. The laptop had to reboot mid-gig several times over the years. I believe that Apple include the best technology they can and I am extremely comfortable with the iPad's headphone output.
If you've got issues with the quality of the tracks I would suggest you look to improve them at source. You could obtain software to try and clean them up a bit. I use Adobe Audition or Audacity which have great filtering techniques - to emphasize or de-emphasize bass or treble frequencies (you can even specify the range). They also have the ability to normalize tracks together so they play with minimal use of the volume button (because one track is louder than others). Look up "normalizing". Am sure Peter will have mentioned it somewhere in this forum.