I don't really know enough about this aspect (ie, achieving mwah!), but logically the nut won't have any effect because the mwah is created as you run a 'fretted' point up the fretboard, and the neck likewise won't. I'm personally sceptical that the fretboard would make too much of a difference, assuming that it is a relatively hard wood.
Action height will and therefore neck relief will - if it is too high, then you won't get the vibration of the string interacting with the fretboard at the 'fretting' point. And string tension will - lower tension will vibrate more and therefore hit the fretboard more. The pickups and /or eq might...piezo's and some other pickups emphasise the higher frequencies better and, logically, the mwah frequencies are the higher ones?
But maybe there are other factors.
If it was me, i'd check the action height, I'd try low tension strings and then, because it is probably the cheaper option, spend a day in a well stocked shop and try other basses. Anyone else got a view?