Congrats on your new Magicos. I've heard the Magico S5 MkII several times, and was always super impressed. No inherent brightness to speak of. Once the woofers get fully flexed, bass should extend and improve, and may even balance out the treble somewhat. New caps of this caliber can be pretty sibilant for a while, but should come around.
In November I installed a couple of new caps in my preamp, then tried to evaluate some DACs....everything sounded shrill. I thought the new DACs were awful, then remember the new caps. Within a couple of weeks they got much clearer, and opened right up....no more shrillness. The DACs were fine.
Best of luck with your acquisition