Unfortunately I have not heard the m6si500.
If I remember correctly, I listened to a MF M8xi at the LA Audio Show back in 2017 paired with some large B&W speakers. Very dynamic.
It’s been my experience that some speakers respond well to / need power to sound their best regardless of volume. I found that to be the case with the ATCs I owned. They sounded their best when let loose, but were a tad less satisfying at lower volumes. That is why if I were to go back to ATCs it would be with an integrated with a loudness feature for low volume sessions.
ymmv

