Preseason is always a good time for test running new/different sets.
Kubiak doesn't usually go to empty backfield and/or vertical sets, even in third down situations. Partially because the personnel doesn't fit (few of the receiving corps are playmakers), partially because he strongly believes in his scheme.
Empty backfield usually only works well in hurry up situations, because you're otherwise leaving your QB vulnerable. As our ZBS has strengthened over the years, they could be taking a second look to see if they can sneak in more 3 deep plays with the youngsters out wide.
“There's a difference between interest and commitment. When you're interested in doing something, you do it only when circumstance permit. When you're committed to something, you accept no excuses, only results.”