I'm doing this for aesthetics and satisfaction. I've seen plenty of builds (mainly European) with a hardwood front baffle and nobody blinks at that. Here is my table of density estimates for Black Walnut, MDF, and Baltic Birch plywood:

black walnut is 660 kg/m cubed or 41.2 lb/ft cubed

MDF is 31 to 62 lb/ft cubed

baltic birch is 580 - 620 Kg/m cubed

So black walnut is below the median of MDF density, but not out of range. its somewhat denser than baltic birch. MDF is essentially isotropic and (I assume) relatively invariant across a given panel. Black walnut will have different properties depending upon axis and some variation.

I've convinced myself that in this compact (12"" x 8"" x 9.5") little box, the effects will most likely be inaudible. What do you think? Please point to objective data if you can. Thanks!!