Try this. http://guitarscanada.com/Board/forumdisplay.php?f=33 . When I tried that amp, I was not impressed at all by it. It was just so fizzy with no low end at all. But onto the real help. You only noticed this after the tube was switched? Try switching it back and see if it goes away. If it does, its the tube, if it doesn't then you knocked something loose.