Hey, I'm here!
The straight dope: the best any EUB could hope to achieve, would be to sound like a double bass with a pickup on it (through an amp). Since the NS Design basses are tightly constructed and are solid-body instruments, their habits lean towards a more electric sounding bass (more sustain, more string detail, that sort of thing.) That's why darker, quicker-decaying strings -- like the Traditionals -- help make it sound a bit more "authentic." There are other options that might work even better for you - Pirastro Obligatos are quite good like this, since they have a gut-like vibe and a less "taut" feel. They have low B and High C strings available, too.
One other thing that might help - NS Design sets up the basses with VERY low action. Too low, in my humble opinion. (It's probably to bolster the confidence of the electric bassists who try the bass when it's on display at your local guitar superstore.) To get that "big" URB sound, the strings need enough clearance to be plucked with a bit of vigor, with the meaty side of your finger, like a "real" bass. Playing with your fingertips, bass-guitar-style, will sound like... well, a bass guitar. So the strings will need enough room to get that "bloom" without buzzing against the fingerboard. I raised the action a little on my personal bass, and that alone improved its sound quite a bit, because I could PLAY it more like a real URB.
Like my sig says - "the sound" comes from your fingers. ;-)
You're probably not going to make it sound exactly like an upright, but you should be able to get within spittin' distance.