IMHO, keep the Santiago's stock. Extensive mods would be necessary to house the 511B's. Remember, they aren't as wide as the 873 Barcelona. That used the 411 down low, with the 511B horn up top, and crossed at 500hz.

I got a chance to audition a prototype 9861 during development in Anaheim for the Japanese market. Left a lot to be desired in the mid/upper-bass area, crossing a 411-8A @1200hz to a 902-8A/811B for midrange, if I recall.