Vudu sexes up its set-top box, but is it too late?Vudu, a startup which sells a set-top box for downloading HD movies over the Internet, has finally added adult content to the mix through a partnership with AVN. Neither Netflix nor Apple will let you watch folks bump uglies — in stunning 1080p resolution, no less. Vudu rival FyreTV won't let you download anything but porn, so it's certainly a differentiator. But is it enough to save Vudu's business model? Unlikely. At $299 (marked down from $399), the box is pricey, the selection of videos still limited, and the premise that viewers will spend up to $20 to virtually "own" Ashlynn Goes to College 3 questionable. And of course, the real competition isn't other paid services — it's the millions of hours of free porn available on the Internet.