My 18" subwoofer was as easy to tonally integrate as the two 12" and the 10."
Using all four subs in the same room essentially eliminated the rooms bass modes and reduced the demand of a single sub. The two crawl test located 12s subs eliminated all but one bass mode in an unused corner of my room. A single located sub should sound fine at the listening position. Your results may vary.
There is no SUBstitute, welcome to the deep end.