I have 2 ST-9100's & both work flawlessly. From the symptoms you describe and the tests you've made with & without the ST-9100 in the loop it's hard to see how the problem could be anything except that the jack somehow got wired in mono. If that is that case, Chris will fix it for you, but hopefully he knows something simple that could be causing the problem we didn't think of. I once had someone else build me a 1/8" female to dual XLR male adapter that displayed the same symptoms and he later told me he must have had a temporary brain cramp & wired it in mono.