Corksheet will dampen some high frequencies since its a soft and dense (when wet) material. This has more to do with muting the resonance of the wooden casing than actually adding lower frequency response. It also gathers moisture rather well. This will gather the moisture in the glue joint between the wood casing and the cork. I have used cork under the reedplates in accordina, glued to the wooden body which also acts as the reed channels. It is manageable, and the accordina is in daily use with a touring band.
Hardwoods are hard and dense, and they reflect high frequencies better. If you want to mellow out the sound or increase the bass register, perhapse try with a slightly larger casing. Also, holes in the bottom of the casing make the sound brighter. Try to think of the case as a speaker box. Perhaps you could experiment with a small reflex tube instead of soundholes or a grille?
This reply was modified 2 years, 4 months ago by Tatu Salmela.