IMHO, my Storm2 is much louder than my DX. I partially attribute that to the size of the earpiece grill. The DX has those sensors in the plastic bezels on either side of the grill, whereas the grill is much longer on the Storm2. I think what you're facing is simply bad(?) industrial design.
Unfortunately, I do not know of a hack to make the earpiece volume louder, so I don't know if it is hardware or software related.