Tell how it's hooked up.

If you have an older HT system you see this a lot and unless you can turn off all audio processing and just use it as an decoder and amplifier then you will never be able to use this old HT.

Today, here's how I hook up a system. Again, I did not research your HT. Supply links when you want that because I can't guess which variation of a model you may have.

Source -> HT system -> HDTV

If you take the output from the HDTV to the HT system, pain seems to follow.
Bob