Actually its more technical than that. With a SEOS18, likely crossing @ 800hz or lower you want to match the directivity at the XO and keep sensitivity as high as possible. Here there just is no substitute for a 15" "mid" (say Beyma 15pb80 or AE TD15M)...well you possibly can go with multiple side by side smaller woofers like 8" maybe, but I am not sure it all works out perfectly well or not. No experience there. But if you want to keep the midrange as pure as possible, it is nice to offload some of the bass demand to the next larger woofer while keep sensitivity up (Erich wants to better the theater monitors that use JBL 18").
JoshK on most other audio forums
Quantity only when quality is ensured.