I'm trying to composite an image into a photo of a television set. I managed to place the image correctly. However, I lost all of the great reflections, etc. from the original plate.

In my scene file, I managed to matte out the screen without the image, but I'm not sure how to combine it with my comp. I have been trying to create an alpha matte with a ramp, but, so far, I have been unsuccessful.

there are some things that need to be added!

I tried to see your hip file, but it doesn't show ur modelling.In order to have a correct tv feeling, there is something which is important, it's the reflection of the tv.
and in order to have it, it would be a good idea to model your tv, generate a dummy geometry. Once you have it, try to render the reflection put on the grid that represents the screen of the tv.
then, the tv that u have as a reference is off. So, you see the screen black.
do u have references (pictures) of your tv on, cos it will allow u to see how tv reacts in reality! usually there is a bit of global illumination to add.

the moment get at least these elements, u could comp them. Do u know any compositing software, like shake? I don't know if Houdini's compositing setting is as powerful!
I've sent u a picture of what I did for my own project!
I hope it'll help

Keep me updated