Hi GS556, I am afraid you are wrong on all counts. A transistor and a tube operate in the same fashion when used in an audio circuit. It is true that a transistor will be off with no signal or bias applied to the base, and a tube will be on with no signal or bias applied to the grid, but they are never used in an audio circuit without some bias applied.
How much current they pass at certain signal levels depends on how they are configured. Class AB push-pull amps, whether tube or transistor, pass a small amount of current at idle and more when a signal is applied. Class A amps, whether tube or transistor, pass approximately half of the maximum current at idle and then this amount varies up and down with the applied signal.
The reason you should never power up a tube amp with no speakers hooked up has to do with how the output transformer operates, not how the tube operates. Energy is transferred from the primary of the transformer to the secondary by magnetic fields. When the expanding and contracting magnetic fields created by currents in the primary windings cut through the secondary windings, they induce current to flow through the speaker. Without a speaker hooked up, this energy has no where to go but back through the transformer primary where it induces a large voltage across the output tubes. This can cause the tubes to arc and damage them.