With respect to bendability, I disagree with Tim, I find GM's much
easier to bend than Special 20s. Although I only have one GM, so
maybe I just got lucky but I don't think so. To me the GM feels
almost like a LO in terms of playability.
There really is nothing in the design and construction of these two
models to make a difference in bending the reeds. Both are handmade
models, so they use the same reedplate specifications, aside from
the external dimensions.
One nice thing about GM's is that the reeds are arced, so its
easier to set them up properly. I think Hohner should do this on
SP20s too.
Again, there is no difference in the treatment of the reeds coming
from the factory. I don't think Hohner adjusts the reed arc at the
factory on any harp; what you get is what comes off the machine.
I've never seen any consistent differences between reeds on these
two models.