I've never played or seen a bass like you described. Walnut necks are very rare, so I can't really comment there. But to talk about the body and fingerboards...
Well, firstly, I would never say that mahogany bodies sound dull. To me, mahogany is a very neutral sounding body wood. Highs, lows and mids in equal amounts. It really lets the pickups, the neck and fingerboard woods and the character of the strings shine through.
Alder is similar but a bit sweeter sounding. Resonant, dynamic, lots of high mids.
And maple...well I've never played a maple bodied bass but I understand that they are pretty bright sounding.
The idea that mahogany bodied basses sound dull is probably down to the fact that old Gibson basses had mahogany bodies. They did sound rather dull compared to Fenders of a similar vintage but you have to take into account the fact that they had those mudbucker pickups and a shorter scale. It's like when people say that bolt ones are punchier than neck throughs and neck throughs have better sustain than bolt ons. Wrong. Fenders are punchier than Alembics and other early custom basses and Alembics and their like have better sustain than Fenders.
There's a lot more in play than just the body wood or the neck construction.
And by the way, Pau Ferro is a very good fingerboard material. Lacks the ping of ebony but very balanced sounding and bright.