I'd drop the 15" and replace it with another bass cab, so three 18" bass and one double 12" and HF per side.
The reflex 18's will play high enough to not need the 15's and I would start out with an xover point around 120Hz and play around to see what you prefer. You may find 140Hz better.
The double 12's will never really have enough low end for three 18's, but you could EQ some 160 to 180Hz in on the 12's to give the impression of warmth. Overall I think the system will deliver a clean sound, but on the smooth and sluggish side. For more dynamics and punch you would have to look at horn loaded bass and mid sections and all the difficulties that arise with these designs.
For general purpose use with smaller audiences close to the stacks, you can't go wrong with an all reflex system.
I currently am running a similar, but somewhat different system to what you are talking about in a small club. I have not been able to build exactly what I want yet so Rob's experience as a builder is something I indeed do respect.
The system I am referring to uses a pair of old Peavey FH118 (circa 1970s) folded horn subs. On top of those I have a pair of 3-way tops (audio Centron), each with a 15, a 10, and small CD horn.
I am using this for live sound and I feel like my results have been phenominal!
I am just biamping with 1760w to the subs and another 880w to the tops. The active crossover is set to just over 100hz sending everything below that to the subs. From 100hz and up to the tops.
The passive crossover then sends 100-400hz to the 15, 400 to 4000hz to the 10 and 4000hz and up to the CD horn.
One other note I might add, I have experimented a bit with using at 75hz or so, and a light duty sub from 75hz to close to 200hz. I liked the results really well. It seems to improve clarity far above where it should...... That is just my experience thus far though.
Remember to take into account what you are building it for, how well can you handle a transport them too.