Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
First off....the TV NEVER CONTROLS the Input or the BOX's resolution.
You either connect it to HDMI for all the Hi Def video, or you connect it with Scart and DON'T GET Hi Def..
Cablewise, it's that simple.
As for 576i and 1080i....576i is Low Def, and if you are using Scart...it's Low Def 576i. Try connecting it to HDMI, or even the Component cable connections because those are the only ones that will deliver 1080.
So now look at the connections you are using on your Humax.-- It's HDMI and you're getting the Humax picture on the TV screen, rigtht?
Then half the battle is won.
The other half we'll continue after a response from you.