Quoted from SkyKing2301:building a box around it can't change the tone, because the bell, striker, and wiring have not changed (as long as the box doesn't touch the bell itself. The only effect of the box is a reduction in the "amount" of sound that can escape, hence, a reduction in volume of the sound reaching the player.
It doesn't change the tone of the bell inside the box, but it would act like a low pass filter and change the tone outside of the box. Sorta like when your wife is hollering at you for not doing something from behind a closed door in another room and all you hear is: wha, wha, wha.
I don't have a great suggestion resistor size. If you put it across the lugs, bigger = louder. If you put it in series, bigger = softer. You could try - Just perusing a Ship Ahoy schematic, I see that they use a 75ohm resistor in series to step down to 6v for a light, if that helps any.