I think you have the wrong TV Tuner model number. A Google search for "AV727" mostly results in Avianca Flight 727.
Your set top box may be capable of outputting 1080i content, but that's going to depends on which output you're using. The analog, coaxial output (which is what you're likely using to connect to your TV Tuner card) is likely only outputting 480i.
Since you do require a set top box from your cable company, I'd recommend looking into a cablecard solution to replace your current TV Tuner card AND set top box. Something like the Ceton InfiniTV6 or SiliconDust HDHomeRun Prime.
-Wolf sends