Rubber Bass is sampled from a funny little Kala ukulele bass. It's about the size of a kid's guitar and has big fat rubber strings which are a nightmare to tune.
It's not very loud acoustically but when DI'd, it has a wonderful round, rubbery bottom end with a lovely hint of twang which belies its tiny stature. Its a little bit double bassy, but essentially has a character all of its own. It's not likely to be your go-to-bass sound, but for certain things it is bloody marvellous.
Its sampled with eight layers of round robin of every note and a layer of adjustable note off finger release. There's also four pages of Kontakt scripting, including an arpeggiator, which is really great for quickly playing throbbing basslines and the organic-ator brings a bit of humanity to your sequencing.