also i forgot to ask. do power amps interchange with all cabs or would i need a special one. i doubt that i would but i'm just making sure. i was looking at a qsc rmx850 which is only $300. it sais that it delivers per channel: 200W at 8 ohms, 300W at 4 ohms, and 430W at 2 ohms. how do i determine the ohms in my cab if my cab is 400W. thanks