Welcome to the forum!
The 1019 (or any receiver that I know of) will not convert an HDMI video input into any other type of video output. So, you won't be able to use the HDMI video until you have a TV that can use it.
You can certainly use the digital optical audio connection between the cable box and the 1019 regardless of how you have the video connected - so go ahead and do that. You'll just need to assign that audio input to a source selection, but you've probably done that given that you know it works.
Does the cable box have component video outputs? What type of video connection were you using to get a good picture when you had the cable box connected directly to the monitor?