Part of the mwahh comes from the windings touching the fretboard.
Some of it comes from the fretting hand allowing the sting to buzz directly under your finger (really light touch)
The action should be really low with a very straight neck.
Flats can mwahh but they need to be flexable, the mwahh, again is the string coming into contact with the fretboard after the note is plucked.
I personally really like tapewounds especially light Guage fender nylon tapes, CHEAP but they don't make 5 string sets.
Plucking ON the fingerboard helps (near the heel)
Lower the action as low as it can go, pluck lightly with the right hand at the heel of the neck, and finger the note lightly.