[sundown x18s what kind of enclosure should i build? and what to tune?]

If you figure 2.5 cubes per 18" in a 2:1 ratio that's 15 cubes plus displacement. Optimally you need about 22 gross cubes for a 4th order. What are your dimensions?

[sundown x18s what kind of enclosure should i build? and what to tune?]

27 isn't too low.

Here is a video of m 4 X 18"s

low note on this song is 22hz

http://youtu.be/k73xq6vjjn0

[sundown x18s what kind of enclosure should i build? and what to tune?]

The x series do really well in 4th order enclosures. About 2.5 cubes per sub sealed in a 3:1 ratio tuned to 40hz. You'll have no problem hitting 10 hz (if that's what you want)