Either the UM18 or VBSS option could would great. VBSS will likely have more output up top, UM18 will likely have more down low.
Personally, I'd do a sub crawl with the measurement mic and your existing subs, see what kind of response you get, and plan from there. If you want to do 4 subs the VBSS is affordable, or you could add more UM18s in the future. And I'd build custom boxes to fit in the space you have as best possible.
Realistically, two or four 18" subs, whether UM18 or PA460, are going to blow those Dynamos out of the water. I wouldn't really worry about integrating them.