Check your bias! I bet the bias is off. What you mention about the tube flickering and distortion is what happens when you install new different final output tubes. get a digital multimeter, a small flat head insulated screwdriver and adust the bias to -.0040v per tube. From my experience if you want a little more base you can run the tubes a little cooler, like -.0035v If you run too cold you will loose the crisp midrange and highs. too low and you get the results you described, too high and your tubes will glow brightly and live a short life. You need to check your bias every 2 years and whenever you replace the final output tubes.
