SS subs are touted with having amazing SQ and are known to get very low. i have not personally heard the 8 and have only heard the 10 for a few minutes(bad memories, it was stolen).
i own the 15 and it is a sq beast. someone else told me who has owned the 8" and 15" says the 8" is the same great sound, just less of it.
hey if u buy a 10 and dont like it, sell it to me =D