A: Make sure that this stays a hypothetical question, please! It is very important to follow Boogie’s rule on this one: the bias setting must match the tubes in use! Tube and/or amp failure can occur quickly if this switch is not set properly.
EL-34 tubes run hotter than 6L6 tubes. If you install 6L6s and set the switch to “EL-34,” you’ll cook the tubes and possibly other components. If you install EL-34s and set the switch to “6L6,” the EL-34s will be underpowered, wimpy, and thin.
Don’t do it!