Hi Scnoodle, as you have non-PVR STBs, you could check to see if you have the "record to VCR" option, if so here's what you could do; be aware that you will be watching what you are recording unless you split the cable and send one feed to the TV and watch a basic channel off the TV tuner while recording. You may also want to check your over-the-air capabilities in your area, this way you could at least watch the local HD channels while recording stuff otherwise it's plain SD basic cable through the TV.
First things first, always set an appropriate recording mode for your recording ex; 2-hour movie or little bit less, set the recorder at the 2-hour mode and so on, it's not really recommended to go past the 2-hour mode as you loose resolution, these are to the eye of the beholder, so some testing on your part to see what is satisfactory to you.
1- Connect the S-Video with audio to the Mag input.
2- Set a "Record to VCR" timing on the STB
3- Set the same timing on the Mag (line-1, I suspect) maybe a minute before the start and a minute after, check the clocks on both PVR and Mag to be as close a match as possible.
4- Change the TV input to its tuner to be able to watch basic channels (or OTA) while the Mag is recording off the STB.
I work with a PVR, I record my subscribed channels on the PVR, so sometimes I'll record as stated above and/or here's how I (and many others) also operate;
1- the PVR's S-Video and audio outputs run into the recorder's line input
2- Record all I want on the PVR's HDD
3- When no one's using the TV ot PVR (at night or off shopping) I'll start the one-touch-recording on the recorder for one setting longer than the PVR recording or I will actually set a timer recording to start right then and there for as long as the PVR show, wait for the recorder to start recording, I sit for about 15-20 seconds recording essentially nothing then I start the PVR recording, I do this to be able to do a nice edit afterward splitting the front end of the recording and the back end, tilte up the recording and dub to a disc.
If you are going to record and watch TV at the same time I would suggest to tuck away the STB's remote while recording and use the original TV remote to prevent the GUI to pop up by accident, believe me nothing more frustrating than looking at a nice recording and all of a sudden you realize that you pressed a couple of buttons on the STB remote and it shows up on the recording !!!