Reedthis, there always is some level of adjustment that the manufacturer can make in the operation of the tubes in a given circuit. All "biasing" does is change the voltage at each tube. If the designer included some potentiometers on the circuit board to allow for this, it would be easy for a technician to optimize the voltage for the tube. It is not something an end user would do, typically. For most preamps, this tuning is done with the choice of resistor values rather than a pot anticipating that a tube of a certain type (with certain electrical properties) will be used. Thus, we don't talk about "biasing" a tube preamp, but it's certainly possible to make an ajustment in the voltage being applied if one is prepared to make some adjustments inside the case.
.
