Just to bring some kind of conclusion to the subject of trying to achieve that elusive double bass sound. I ordered a Harley Benton u bass from Thomann and must say I'm really impressed with the guitar. It is a little larger in size to the Kala and as such feels so much easier to play and the bottom end is really stunning, here are the stats.
Harley Benton Kahuna CLU-Bass Ukulele Custom Line Kahuna Series Uke-Bass, Traveller-sized, spruce top,
mahogany body, carved back, mahogany neck, C-shape, blackwood fingerboard (Pinus Radiata), DOT-inlays, 20
frets, scale 534 mm, nut width 42,0 mm, blackwood bridge (Pinus Radiata), black open gear tuners, pro pickup
system with built-in tuner, Aquila strings, finish: natural satin.
At £130 it isn't expensive and really worth trying out, I will be using this bass at our next practice in two weeks time "cant wait to bang it through a 300 watt rig" and look forward to the bands hopeful surprise at the fantastic low end from such a small instrument.