I’ve always liked this christmas song, especially finding the right key for it: in my opinion, it needs to be very deep, very mellow, just shy of the point where it gets too deep and you can’t put any chords below it without it sounding stupid. And every time I try, I arrive at B-flat as the lowest possible key.
Also, my keyboard can do strings too, and I enjoy the sound of that as well (especially how it starts sounding like a church organ when you just keep holding down the pedal and adding more notes – see the end of the recording!), so there’s the second half for you. (Yes, I am aware there’s nothing „leise“ about that. I still think it sounds nice.)
Same thing in A-flat instead of B-flat. See what I mean about it getting too deep, where the chords start to sound weird? The lower you go, the less notes you can have in the chord.