Many thanks to both of you for your helpful comments. @Snargle -- very informative link!!!
The luthier of this fir-top uke presently has 2 ukes available, both are tenors with all solid wood -- top, back, & sides.. One has a Port Orford cedar top (Lawford cypress) with Hawaiian koa B&S. The other has a D. Fir top with claro walnut B&S.
Via e-mail, I asked the luthier to play the ukes & answer the following questions (based on what his ears heard):
a) Which is has a fuller bass?
b) Which is mellower/warmer?
c) Which has the longer sustain?
d) Which uke gives your tummy the better massage?
I will quote-in-part the luthier's reply:
a) The walnut and fir is better in all your categories. It just came out better all around. It has a mellower sound with a fuller bass.
b) The walnut and fir also has more vibrations felt on the back side while playing.
c) They both have equal sustain in my opinion.
d) I find the claro walnut has a really good well rounded sound, and the douglas fir
produces a highly resonant instrument.
e) In the words of my supplier, "It was
a real surprise to find that Douglas fir was actually four times as
resonant as Sitka Spruce. The sound is warm, yet crisp and clear at the
same time with excellent harmonic development."
As for me (bellgamin) I take the supplier's "four times as resonant" with a grain of salt but I feel the luthier's comments are sincere.