FOR iTUNES:

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.

How to setup your MacBook

FOR SPOTIFY:

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."

If you plan on using a MacBook for your music player here are some suggestions on how to set it up so that you have a worry free experience:

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Using Spotify

Set volume on iTunes and system to 60%
Set system noises to off.
Set system to not standby.
Set screen not to sleep.
Turn off Airport
Turn off ancillary programs

Using iTunes

How to setup your MacBook

If you plan on using a MacBook for your music player here are some suggestions on how to set it up so that you have a worry free experience:

Check out Just Press Play's Services in your Area...

Set volume on iTunes and system to 60%

Set system noises to off.

Set system to not standby.

Set screen not to sleep.

Turn off Airport

Turn off ancillary programs