Stupidly, that link actually doesn't explain how to have the remote control the AV power. I don't understand why TiVo still has this info not publicized.
To make your remote control the AV power, hold down the tivo+power buttons for a few seconds until the light comes on, then enter in the AV code again (the same one you used to let you control AV mute and volume). That's it!
Note that, if you have a choice of many codes, a code that works for mute and volume may not work for power - you may have to keep trying other codes for your brand receiver to find one that works for all three.
Thank you. I had to try quite a few codes but I finally can turn my Onkyo receiver on and off with the Roamio remote.