This is in the FAQ, but there is a lot of stuff to dig through there. I edited and expanded it a bit so it ought to clear up a lot of the myths surrounding tube amps and speaker loads:
To understand WHY tube amps don't like higher impedances and why they are MUCH more dangerous to tube amps than lower ones, you have to understand what it is in both types of amps that actually does damage.
In solid state amps, the output transistors are connected directly to the speaker load. Amps put out a constant voltage and the varible that determines power/current is the impedance of the load. Ohms law: P (power)= i (current)/r (resistance). The lower the load (resistance), the greater the amount of current is passed.
Current also generates tremendous amounts of heat, which is why if the load is too low, the heat stress on a solid state's output transistors can damage them. The same rules apply to tube amps, but there is a deciding factor that makes a huge difference. Tubes are extremely high impedance devices, and in order to be connected to a speaker, must be coupled through an output transformer which takes the high voltage/low current output of the tubes and makes a high current/ low voltage output that can drive a speaker. Instead of being constant voltage amplifiers, they try to be "constant power" amplifiers; each tap of a tube amp's output transformer is wound at a voltage/current ratio to supply equal power to each of the taps of differning expected secondary impedances.
A transformer is an inductive coil that works by amplifying current and/or voltage. If the transformer is not connected through a circuit on both its primary and secondary taps, it will feed back on itself and generate a large voltage spike at its connected end. What happens with too high a load connected to the transformer's secondary is that it acts like an open connection and causes feedback within the transformer. Since the primary of the transformer is connected to the plates of the output tubes, this voltage spike hits them first. As long as it doesn't exceed their limit, you're OK, but if it does, you can short them, which WILL destroy your amp.
The reason a lower impedance is not so dangerous has to do with the nature of the tubes themselves. If the transformer secondary is connected to a lower than expected impedance, the transformer obeys Ohms law and tries to maintain a constant current to a lower load for an increase in power, but this causes a drop in the voltage of the plates of the tubes and they emit less, so power actually decreases, but because more current is flowing through the tubes, they get hotter and "age" their cathodes at a much faster rate than normal.
