Amazon is gearing up to take on Apple and Roku, again. Industry sources say Amazon is getting ready to launch a Web TV box that would compete with Apple TV and Roku’s line of products, which make it easy to move video from the Internet onto your TV. People I’ve talked to who are partnering with Amazon believe the company is aiming for a March rollout. That won’t be a huge shock, since Amazon has been working on a box for much of last year. The company had planned on launching one in time for the Christmas shopping season, then shelved those plans. A box is also a logical move for Amazon, which is investing heavily to build up a Web video catalog, and is starting to produce its own shows as well. An Amazon box will allow its customers to easily watch that stuff on their TVs; it should also give other content providers a chance to serve up their stuff via apps, in the way that Roku and Apple TV boxes do.