IMO, the mwah - which I
love - comes mostly from the interaction of the strings, fingerboard, and fingers. The Body woods, pickups, and electronics can enhance it or interfere with it, of course. If you get a loater model, chances are good it'll have the Thunderguts module - you can also have Mike add one - which can put you out in front with a nice singing mid-rangey tone for melodic solos.
Picking up fretless really opened my ears to a lot more tonal alternatives and encouraged experimentation with different techniques and even toys (normally I'm a purist, but I had a good time trying to recreate Pino's 80s tracks!). I don't sing, but sometimes I'll use the fretless to fill in vocal harmonies, like in the a capella break on Zep's "The Ocean"; people who are paying attention can be kind of amazed...
Good luck & keep us informed!
Richard