I've been following this thread for quite a while now. I have been using Comcast/Xfinity Motorola boxes with a Denon AVR-891 receiver and Samsung plasma TV. HDMI is a real nuisance. Glad to know that using a powered splitter can work around the issue, but that doesn't really suit my needs. I wish Motorola would fix this already... their design leaves much to be desired.
My cable box has multiple active outputs: HDMI to the receiver (and from there to the TV) and component/TOSLINK outputs to a DVR. Using an RNG110 box simply didn't work: the two sets of outputs conflicted with each other and usually wouldn't work simultaneously. I'm presently using an RNG150N with no problems except for the HDMI switching issue. When I turn the receiver on before the TV, the video setting in the service menu reverts from NATIVE or 1080i to 720p. The same problem usually happens if I simply change the input setting on my receiver to something else (e.g., CD), regardless of whether the TV is on or off.
I just tried a DCX3400. The only difference I've found is that with the box set to 1080i in the service menu, changing the input on the receiver no longer reverts the setting to 720p. This still happens if the box is set to NATIVE, however (or if the TV is powered on after the receiver).
Also, the DCX3400 froze while in the special menu.
I especially dislike the fact that this drop in resolution also affects the component video feed from the cable box! If not for that, I'd be able to avoid this problem most of the time. I'd use the HDMI feed only for 3D content and when I can't pass video through my DVR.
Unfortunately, I'm also having problems reliably recording from Comcast to the DVR. But that's another story.
Some good folks at Comcast are trying to help me get somewhere with these problems. It's slow-going, but I remain hopeful.