I've heard some wonderful fretless soloists use a nice wide vibrato and I'm wondering how it's done. I know there's the typical side to side string stretching technique guitarists use. I would like to roll my finger forward and back like the upright players do but I don't get enough pitch change. I can't slide my finger along the strings without dampening the vibration. Maybe I need a different kind of string. I'm currently using SIT strings "silencers" semiflat 45-65-80-100-120 on my five-string carvin fretless. What do you recommend?