I was looking at a case the other week where there were two SC5000s and an X1800, all vertical (with the case being flat on the floor, the open side upwards.
I think the 1660 should fit. While bulky, it obviously (like the 1615 and 1650) has trolley wheels and a handle. So transporting it around airports and such shouldn’t be too much of an issue. And you can always let your cab drivers lift it in the trunk 
Pluck-foam will allow you to shape the compartments to exactly fit your gear.
The 1615/1660 are 23,5 or so deep. With decksavers you are looking at ca. 15cm for the 5000s and 12 for the mixer. Together 27cm. So I have a hard time imagining how you could stack the mixer on top of the players and still be able to close the box. And that is without any padding between the two.
I am not a great graphics guy, but it would look something like this:
