i generally tune 30-34 for 12s and 34-38for tens based on the fact that the larger cones reproduce lower notes more prominently or with greater ease but this is my general ways. u can tune to 30hz with a ten but it will be louder tuned higher other factors can change this but again it just a general way. my two z12s are tuned to 30hz and i love them but when i tuned my dd 2510 to 38 it was depressing i picked the tuning up to 40 and it played down to 30hz fine but got alot louder for everyday music.
i cannot speak on the sealed situation i have no experience with sundown and sealed but i have found though a number of subs that .8 cubes works great for tens and allows them to catch the lower notes a bit easier.
all in all i still recommend getting one of the design guys on here to design a box specifically for your vehicle, it might cost u a few bucks but they know their stuff and have more experience then me. ramdesigns, mobileenclosures, and prorabbit are a few of the guys i suggest. buck used to do it for a buck lol but idk if he still does pm them for more info