You could get distortion for a number of reasons, however, I would not assume it is the Sunrise that is at fault. I have pushed my Sunrise significantly past 12 o' clock with not resulting distortion. I have read reports of the Sunrise being quite good with the HE-6 which, of course, are much less efficient than the HE-500. Try the sunrise with different headphones and different tubes. Alternately, try the HE-500 with a different amp.