I'm not sure exactly what you mean by brightness. I don't think the 7Xs color the mid-range or low frequencies (although there is more mid-range there). The highs are more additive. Something like a high cymbal comes through clearer and stronger. If you are listening to audio with highs you will hear it more. You may or may not want this. In my limited listen the 5Xs were really good. I personally am happier with the 7X in both mid and treble range but I can see where the 5X sound might have a strong appeal to others.
Thanks for sharing. How are they for extended sessions?