I live in Marshalltown and just installed an amplified outdoor mulitdirectional and I get a ton of channels. Including Cedar Falls, Waterloo and Mason City. That said, I'd bet you could get those channels with a regular set of rabbit ears. Like the earlier poster said, try a cheap set of rabbit ears first and if that doesn't work move up to an amplified antenna.
Oh, and the fact you have cable doesn't matter. But, you might need to get a switch so you can switch between cable and the antenna. With dish you can hook the antenna up to the dish box but with cable, I doubt you have a box to hook to.
