They are 8 ohm's each. So you could wire them in parallel down to 2.66 ohms, but keep in mind that will result in mono, (not sure if those horns are stereo or just mono (like a sub). So you would be looking for an amp that can do 450w @ 2.66ohms x 1ch.
Without doing the math again, I remember, 2.66ohms is right in the middle of 2 and 4 ohms. So if you want something that will do 450w at 2.66ohms on 1ch, Just about any amp that can do a combined 900w at 2 ohm and 4 ohm will do it. (i.e. if it will do 350w at 4ohms & 550w @ 2ohms, or 300w at 4 ohms, and 600w at 2 ohms, and it will work). You can always go higher and turn down the amp.