NS Electric Traditionals - made by D'Addario, they are a version of D'Addario Helicore Orchestra upright bass strings that are made shorter, specifically to fit the NS Basses.
They have a similar (spiral wound) construction, gauge, and feel to the Spirocores, but have an (intentionally) darker, less defined sound - which, considering the tight construction and solid body of the NXT, sort of "balances the see-saw" and helps to bring the sound of the bass back towards a more acoustic vibe. Compared to Spiros, they have less sustain, and less "detail" - which are the characteristics that tend to make the solid-body bass embrace its "electric-ness."
Other, non-NS options that have worked for many players are the Pirastro Obligatos and Jargar strings; but both of those have a more different "feel" than the Spiros.