The best everyday source is indeed the DAB+ radio (I don't know how this works in North America though but here in Europe we can switch between classic analog FM radio and digital DAB signals) which has a CD like audio quality and the sound is great.
If you want to use an usb flash device/SD card/"homemade CD" you can only use MP3 and WAV files, make sure to use at least a 192 kbps mp3 preset or preferably a 320 kbps one. If you have WAV files also choose the ones which have the less amount of compression if you want the highest quality.
If you have the Mark Levinson system there's an alternative if you want to maximize the audio quality, and it's to connect a (decent) audio player to the AUX port through an AUX-IN cable - it can be an iPod or an HD portable audio player - and stream the music using uncompressed files as source on the device, like FLAC for instance or a FLAC converted into M4A for the iPod. With the iPod connected you'll have the controls to switch song etc. but I'm not sure if you'd have the same possibility with a non Apple product, should this happen you'll have to use the physical buttons of the "mp3 player" to switch song.
However, if you don't have the ML audio system you'll be fine too with the bluetooth audio streaming, of course the quality would be a little worse than USB/AUX but if you aren't an audiophile probably you wouldn't even find this out. As I said, make sure to stream high quality files, compressed or uncompressed, and if you're using apps like Spotify and such be warned that you have to be a paid subscription member to have the highest quality for the music you'll be streaming (320 kbps MP3 for Spotify).
At last, if you want the classic portable high quality we used to have back in "the old days" nothing beats the old CD.
Ps. I was reading on an a local automotive news website that the new UX multimedia system will read many other formats such as FLAC and OGG (the latter can be uncompressed) and this is good news for me because I'm getting that car in march '19.
