Hi guys, I have a question regarding my old-version Bogner Shiva EL34. I bought the amp more than a year ago secondhand and it is time to change my tubes. The Bogner site states i should only use the SED powertubes. Does this mean i can just drop them in and forget about biasing? How much difference is there between the SED tubes and why should i bias if i'm using the same type of tubes? Please explain!