It´s the first time I´m getting strings for my fretless JB and I was hoping you could tell me which strings are better to get me some of that "Jaco" singin´bass sound. Thanks.
well...i have to say that im a big believer of each bass should have different set of strings for example i have 3 basses and i use 3 different sets - for my jazzbass i use ken smith (tried them all and this one suits best), for my pedulla fretless i use Labella Slappers and for my faim (warwick look a like) 5 i use d'aderios xl. i say you should try couple of sets : Labella Slappers Thomstik Infled Ken smith and DR. all of course nickel cause stainless steel would damage the finish of a fretless neck. and see what set feels the best for you.
Two factors that contributed to Jaco's sound (besides his hands and his musicianship) are his use of roundwound strings and a finished fingerboard. Jaco finished his own fingerboard with boat epoxy. The strings listed above are good. You could also try GHS Boomers or Progressives (my personal choice), or Rotosounds, which is what Jaco used.