Set your iTunes Settings to your liking under edit/preferences/playback:
Auto Crossfade, if you want it to fade out from one song and into the next.
Sound Enhancer, if you want your music to have more lows than highs or the other way around.
Sound Check, automatically adjust song playback volume to the same level.
Just Press Play Productions
Set your Spotify Settings to your liking under edit/preferences:
Music Quality, Turn on High Quality Streaming(Premium Only)
Click on Show Advanced Settings
Playback, Set the amount of Second under "Crossfade Songs" ( I suggest 7-8 Seconds). And TURN ON the "Set the same volume level for all songs."