Rix, the stiffest enclosure would be a sphere. But that is difficult to produce. Next is a cylinder or tube as you suggest. This is a much easier design to deal with and perfect for a balanced force system. Just put a driver in both ends. I'm building 4 of them with 12" Morel drivers. Two in each cabinet.
For point source systems crossed over below 80 Hz it really does not matter where you place the subs as long as they are spread out always against a wall or in a corner.
For line source systems crossed over above 100 Hz you have to consider the affects on image and the increased power projection a line source has. The best way to do this is by creating subwoofer line source along the front wall. It is still a "swarm" system of sorts just with a more specific radiation pattern.
For point source systems crossed over below 80 Hz it really does not matter where you place the subs as long as they are spread out always against a wall or in a corner.
For line source systems crossed over above 100 Hz you have to consider the affects on image and the increased power projection a line source has. The best way to do this is by creating subwoofer line source along the front wall. It is still a "swarm" system of sorts just with a more specific radiation pattern.