Finally got the cable working yesterday, after scheduling a tech to come out to the house. (That itself was a disaster--Comcast scheduled the tech, then the web said the call had been canceled, then the tech showed up an hour early, then three hours later a different tech showed up...)
Anyway, it turns out the "defective" cable box was fine.
When we got cable Internet, the tech installed a cable TV trap on our line to keep us from pirating cable, but forgot to log that he'd done so. And the cable box we were given was actually logged in Comcast's Babbage engine as being assigned to someone else. Between those two things, trying to get cable working in one day was a non-starter.
