Rather than looking for a blind answer, why don't you look through some of the more thoughtful postings on the 'net regarding subs. I can tell you that an 8" will outperform an 18". Will you believe me?
Teach yourself a bit about subs by going to the following link, then perhaps you can teach others:
http://www.diysubwoofers.org
Teach yourself a bit about subs by going to the following link, then perhaps you can teach others:
http://www.diysubwoofers.org