Biasing is a DC voltage that is applied to the control element of a tube, ie the grid that sets up the nominal starting point on its characteristic curve when there is no signal imput. The incomming signal overrides the bias voltage increasing it on it's positive swing and decreasing it on it's negitive swing. The bias controls both the amplification and linearity of the tube stage. When you have an amplifier using two output tubes in a push pull configuration it is important that the bias on both stages is equal so they have the same amount of amplification to the incomming signal. This is what is meant by a balanced output and it reduces distortion caused by an unbalanced output. An explaination of a pushpull output stage is as follows. You have two tubes feeding the oposite ends of the primary of the center tapped output transformer. The secondary winding feeds the speakers. With no signal input to the output stage both tubes should be either drawing no current or equal current if the are properly balanced (biased). As the incomming signal goes positive one tube conducts more and the other tube conducts less. As the signal goes in a negitive direction the first tube now starts conducting less and the second tube more. As you can see the tube conduct inversely from each other. When one tube is increasing conduction the other is decreasing in conduction, and that is what is meant by a pushpull output stage. Even though the tubes seem to be subtractive, through the output transformer they are additive and greatly amplify the incomming signal.
