I have a Sirui G-20x head, and find that everything almost locks up by -26C, and so becomes almost unusable. The knobs become so stiff that I can't turn them anymore. I'm thinking the aluminium ball may have a slightly different thermal expansion rate than the body. Or maybe there's a lubricant that's freezing? In any case, it doesn't work so good at -26C. Can anybody recommend a head that functions at low temps?