I believe what you mean by Crossharp is the Hohner Crossover – that harmonica is basically a Marine Band with the cover plates more open in the back and put together with screws.

The Special 20 differs from the Marine Band in two important ways –

1. The plastic comb of the Special 20 has the reed plates recessed into the comb so what touches your lips are the plastic face of the comb and the cover plates. A Marine Band harp has the reed plates exposed so what touches your lips includes the ever plates, the comb face, and the reed plates. The plastic in the special 20 absorbs some of the vibrations of the reed plates so you don’t feel the vibrations as much on your lips. How much of a difference this makes in the sound I can only speculate on, but it probably makes a difference.

2. The Special 20 has cover plates that do not have vents on the side. This is a lot more significant than how open the tabs are at the back of the harp in my opinion. If you drill holes in the side of the Special 20 cover plates, the harp will sound progressively brighter. You can also adapt Marine Band cover plates and put them on Special 20s.

In  my opinion, opening up the back of the cover plates helps out a little with projecting sound, can make the cover plates stronger if you do it right, and makes the harmonicas look cooler to boot. But the vents make the most difference in the sound. You can run with this info in a couple of ways: adding vents to Special 20 cover plates one way or another, or closing off vents in Marine Band or Crossover cover plates with tape.

As far as the comb factor goes, the main difference to me is that the Marine Band type harp feels more alive to me cause I am getting the vibrations directly from the reed plates. It may or may not be louder than the Special 20, but it seems that way to me.

Hope this helps!

Richard