Tone comes from the body woods, strings, electronics, cable length, pickups, bridge, tuners, fretboard wood, neck wood, number of bolts used (if a bolt-on neck), effects, amp, cabinets, humidity, wind chill, air pressure, temperature, fingers, amp power cable, fairy dust, angle at which you hold your bass, fingers, picks, thumbs, bows, and nut material. Frets are moot.
This is common knowledge.
I have to agree. I did a full 4 hour gig recently with a fretless Jazz (I normally play a fretted Jazz) and none of the other band members noticed.
At the end of the gig, they were amazed that the sound to their ears was the same.