Looks like a pretty old thread, and not exactly the same problem, but I found it Googling a problem of my own and figured I'd leave my solution here just in case it helps anyone else who finds this thread with the problem I was having.
The same cablebox (DCX3501-M) was outputting 480p to my new HD TV (Seiki LED) and I couldn't figure out where to change the output.
So here's what I finally found, with the TV on, and tuned to the cablebox's HDMI input, turn the cablebox off. That's right, power it down. Then hit the 'Menu' button. Here you can fiddle with what signal the cablebox is outputting over the HDMI cable. My guess is my TV's firmware was sending some bad info back to the cablebox.