one more "bias" question......

[Davis]

Okay, I called the gentlemen at Groove Tubes yesterday, and he said you can change pre amp tubes anytime you want because they don't need to be rebiased. However, he said you could cause serious damage to your transformer (output) by switching power tubes and not rebiasing. So my question is: with these new AD amps from Orange, which I proudly own, can I take out the existing EL84's and pop in some other brand of EL84's without rebiasing?
Also, are these truely "class A" amps? If the answer to both of these questions is yes, I'm gonna have some fun this weekend trying out different tones by playing with different tube combinations.