I replaced the preamptube of my hartkehead with a tube from Tesla. When I tried the amp out in my bedroom, I didn't notice much difference, but at rehearsel........more growl, warmth, punch....... And this costed me only 20 euro's! So everyone with a hartke head, change the preamptube (my tech says that the stock one really sucks!) and for 20 bucks, your sound is much better!!