This is an old project... just though I'd do a quick update. It didn't work at all, really... I was working on the assumption that the air moved by the strings would generate enough sound to resonate in the chamber to make the bass sound, but but no good.

Rev 2, if I get to it, will have one end of the strings attached to a movable surface (green at the right side of the image below) at one end of the tube so that the mechanical movement of the strings can be more effectively translated to acoustic energy. Think strings anchored to a 6" speaker cone at one end of the tube with the cone resonating air in the tube...