Problems with loadbox

Hi! New to the Nano Cortex, but so far, I’m very happy with it. I’m a Marshall guy, and I own a old -69 small box amp that sound fantastic and it would be greta to capture that! The setup I have now for the Nano Cortex is that I use it together with a power amp and then through a Marshall 2x12 cabinet, so I don’t use captures that are done with speakers. I have a old THD Hotplate attenuator that I use with the amp, and it has a load box function and a line out, but when I connect that with the Nano, I get some kind of self oscillation, don’t know why that is happening but when I connect the line out from the hot plate directly to my sound card, I do get a more normal signal. Any ideas why I get this problem?