Update!

URC is the BEST. They reached out to me based on this thread and gave me some great advice.

Their new(?) built-in code set, "Cable 195," works on both my MX-850 and MX-800 for all functions except:

POWER -- this must be a unique PWR-ON code in XFINITY-land as it will not power-off the box. I was unable to teach the remotes this finction from teh Xfinity remote at all, so at least we can turn stuff on now.

TV-ON -- Doesn't work, but I successfully taught this function from the Xfinity remote, so no biggie at all.

I am WAY happier now.

Thanks so much, URC, for your help!

-Gian