american bass gurus please help

[Xplay75]

I plan on making a box for 2 xo 12s but can't find the specs for the max air space,some one told me to go by 2.1 cubes per sub,will this work good,will be over powered,they are in a prefab box now and I'm running a rf 1500 bdcp with gains turned down,the handle how its set up now,I want to tune the box lower,the pre fab box is tuned in the 38 hz,I like my boxes around 32,thanks for any help