the f30 uses an A/B style poweramp, which means that the load is distributed between the 2 powertubes, each tube providing each half of the frequency. In a Class A design, all 4 powertubes are providing the full frequency response, there is no push/pull, so the tube is operating a full bore, no matter what the volume. This means that class a is far less efficient than class a/b, meaning that it will distort sooner seeing as that the tubes are already pushing themselves with your guitars volume off, never mind cranked. variations in bias, the tubes, transformers, and load can also effect the headroom.
