Yes, unfortunately you have to fiddle with all this gear until you find a compromise that works for you most of the time. Large screen LCD displays and DVD recorders go together like pickles and ice cream: the result is generally "blecch"
The problem lies in DVD recorders being tied to the standard-def 480i spec, so the minute you patch one into your system you blow the High Def all to hell. I found this to be the single most annoying consequence of the move from CRT televisions to LCD: in the old days, you just patched your cable or antenna thru your VCR or DVD recorder, and nothing changed on the TV quality-wise. For years, my preferred hookup was to monitor everything on my Trinitron thru the recorder, so I could freely alternate between watching a recording or a live signal. With a flat screen, you can't do that: they stink on ice at displaying standard def passed thru a recorder. You end up having to screw with switches and splitters, as Tulpa said, so that you can flip the TV feed over to the HD source (antenna or cable) when you aren't specifically intent on watching a recording.
I agree this is really irritating. One would think by now the recorder mfrs could have figured out how to put an automatic relay on the output connections, that would sense when the recorder was simply stopped. It should switch to full HD passthru signal when nothing is happening, but drop back to 480i when you start the recorder doing anything (display a menu, play a recording, etc.). Admittedly this would be tough to engineer with any reliability: our new TVs have a hard enough time locking onto a primary signal, never mind one that switches resolutions on the fly. Still, it would make DVD recorders much more attractive and compatible...
Anyway Tulpa's splitter suggestion is the way to go if you are getting hi-def cable service off the bare wire without a decoder box. One line goes directly to the TV as before, while the other goes to the Magnavox. The Magnavox HDMI should connect to whatever your setup requires: usually to the TV, but some setups loop the HDMI thru the a/v receiver to the TV. When you want to watch live TV in HD, switch the TV input to RF. When you want to watch a recording, switch it to HDMI. Sooner or later, your cable company will pull the plug on your "boxless" service and force a box on you. When that happens, you get to rewire everything again. Oh, joy.