In my experience, the shell only makes a very small difference in the sound, with the exception of the Vibrandoneon, where removing the front panel results in a louder and much brighter tone. I bought a Mylodica alto. It looked great, but the wooden shell made no discernible difference to my ears.
Kevin – the plastic shell is removed and replaced with a basic wooden structure. I wasn’t able to remove mine from the case for tuning, as it was jammed in. And the lack of spit valve meant a build up of condensation, which eventually caused the wood to warp. The melodica inside the wooden case was identical to the Angel Melodyhorn 37, and according to this thread, the L-37 is also actually an Angel Melodyhorn.