Advice needed re compact 18Xb sub design

Hi im looking to short cut a project or iggy back someone else's design work. I have several Fane XB18 older colossus not neo magnets... im not even sure if to use them or sell them and buy Eminence kilomax 18s as im not struck on fanes after sales service!
id like to diy build 4 single 18 cabs not too big for transport but efficient and if possible to go down below 40hz to nearer where a low b on a bass guitar is... 31hz! I don't really want to use 2x18 as sometimes we would only want to or have room for 2 - 1x18s but on occasion would double up with 4 which I know would achieve lower frequency than just 2 cabs. Im just looking for the benefit of experience out there in achieving optimum efficiency, depth and portability. ?.Please
Ellwood (we are a touring prog rock outfit with our own PA ... QSC RM5050x2, 2450x2, 1850HDx2 and so on)

Here's a link to the cab  plans Fane recommend for the XB. You'll see they go down to 35hz

Edited by markie - 31 July 2014 at 1:15pm

The B&C sub is 185 litres, I would've thought its better to go with a  cab designed for the driver.

There is rarely one 'correct' cabinet volume and tuning for a driver.  Most drivers suitable for reflex cabinets will give good performance in a fairly wide range but with different trade-offs (sensitivity vs bass extension etc).  After a while you tend to get a feel for what will work and what won't.

Just asking but why do you think the kilomax or say for instance a sigma creates more heat in the amplifier? Thank you