Does your cable box use the new Comcast XR2 remote (on the left) or the standard Comcast one (on the right)?
For the XR2 remote, you would select the "Comcast X1" remote.
For the standard Comcast remote, please try the "Comcast RNG150".
Please let me know if these don't work for you.
Product Manager @ Slingbox
From what I understand about the X1 boxes, the power key does not actually power off the cable box. It turns off the TV and other components (like a sound bar), but the cable box actually remains on. This is why you'll still see video on your Slingbox feed.
I tested this with my Comcast Pace PX001ANM X1 box and found that the LED on the box didn't go out when I pressed power. I later confirmed it with a contact at Comcast.
If you find this isn't the case with the PR150BNM let me know and I will investigate a fix.
Product Manager @ Sling