I'd get back to the modulator's support as well. As you can guess HD over RF is pretty touchy stuff. You may have to try another modulator if Samsung doesn't support such a system.
I'm using a digital modulator to spread through my RF TV network an 1080p HD video source. After tuning all TVs, i can see image and sound in all 48HU7500 TV models but there is a problem in models 75HU7500 and 65HU8500, in this two TVs there is No image. Just sound!
I reported this issue to my local samsung support some weeks ago but i'm still waiting for an answer. Why it is working fine in the 48 inch models and not working in the 75 and 65 inch models? How can i solve this issue?