If you’re after the iconic fretless “mwah,” you’re on the right track with pau ferro. Best as I can tell, the “secret sauce” for that sound is the combination of a hard neck wood, roundwound strings, and a tilt towards the bridge pickup. The most prominent “mwah” seems to come with an epoxy-coated neck, which will be even harder (look up audio clips for Pedulla Buzz basses.)
Ebony and pau ferro are both harder woods than rosewood (pau ferro actually means "iron wood"). I’ve noticed that MTD fretless models all have ebony 'boards, despite their fretted brothers in the same model line having a different wood.
I’ve inquired here on TB about the situation of roundwounds damaging a pau ferro fretboard, and found that it can be a problem if you do a lot of vibrato (read, pressing hard and pulling the strings). However, even at that it should be minimal and easily dealt with by a qualified luthier.
I have a Tobias fretless with a pau ferro 'board, and I went with nickel roundwounds, which get most of the sound of stainless rounds, but are easier on the fretboard. It sounds really nice with lots of “mwah.” I also have an Ibanez BTB fretless with a (softer) rosewood ’board, and it’s much more subdued by comparison. I’m using flats on that one (rounds didn't seem to get more "mwah").
The amount of “mwah” can be dialed in (or out) by emphasizing the bridge (more) or neck (less) pickup. EQ can contribute too, especially treble and midrange. Swapping roundwound strings for flats will tone down the “mwah” as well, as would swapping the neck for one with a rosewood fretboard.
Hope this helps, and good luck with your journey!
Regards,
Wayne A. Pflughaupt
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