The Hohner Melodica Piano 36 is one of the loudest melodicas I have ever heard without being too harsh. Big, full, bright sound. Professional quality. It will easily keep up with horns. You would have to buy one used, however, as they went out of production some time ago.
By the way, I live in near Springfield. Maybe we can hook up one of the days. I am a technical writer that works near Worcester a lot. I have heard about HONK.
I happen to have two Hohner Melodica Piano 36s for sale and both are in great shape. You could try them before you buy one.
There are other options for loud melodicas and I will leave it to others with direct experience to talk about those.
Here is a photo of a Hohner Melodica Piano 36.