My strategy of choosing the type of wood was: “Yay, it fits!” 😉
I’ll try to play the scientific guy here (yap, I’m doing many things) –
First, it changes the way the instrument projects its sound.
Instead of mostly sounding through a single hole (at the open keypad) which tries to force a plastic box to vibrate (not the best medium for sound transfer), it now has the wood vibrating relatively equally through all its length.
It’s the same as letting a tuning fork vibrate in the air, or attaching it to a wooden part (try a piano or a big table); same energy source, way better sound transfer.
It also changed the sound reflection pattern of the instrument. I.e., without the wood, I didn’t like much the sound when recorded from the keyboard side. With the wood, I really do.
I agree about the change in the frequency domain, which may be different using (any type of) quality wood.
To me the sound is fairly stable and constant through all its range, no spikes or strange notes.