This is an exciting speaker, and forum. I have a pair of Gauss 18"s that I have been looking for the right enclosure for, and my search might be over. Is there a need to scale up the Imperial for twin 18"s? would it be more "flexible" for movement and placement to build 2 boxes, one 18" each, or does the need for output necessitate putting them next to each other anyway? thanks

the sheer size of the imperial is its strength in output, its a backhorn, so the length of the horn, and the size of the mouth determine the output.  Above the tuning of the horn, the drivers front wave needs to take up the slack.  Two full sized imperials with 1 18" each would theoretically have +3db over a single imperial with 2 18" in the tuning reange of the horn, and the same output from the frontwave.  This is because all thats really changing in the system is the inclusion of another backhorn, as there were 2 18"s in the comparative scenario.  If you have the room build two, if you only have rrom for one, build one, dont split them, as you will be halving the horns volume and raising its tuning frequency,.