Welcome to the forums :-)
I'm very very sorry... I have just deleted your thread in the General sub forum by mistake! 8-(
It's not possible for me to un-delete it... :(
Would it be possible for you to re post it (once this time) in the Logic or Gear sub forums please...?
From what I remember from your question... I think you should unplug your audio interface and monitor the GB songs in question and other Logic songs through headphones. Then repeat the test using headphone via your audio interface. Any difference?
Also, check that the GB songs' audio files have not been limited or normalized in any way...
Otherwise if only certain GB songs sound bad through the interface, then it would point to the songs themselves not being mixed well?