Tube amp power rating question.

Is it true that the output power for the tube amp does not depend on the load? Here is the response from one maufacturer when asking about the tube amp rating on one of thier SET amp:

"Rated power of a tube amplifier remain the same at any output impedance taps unlike solid state types."

Is it true? If so does it mean I will get the same power output regardless if I use 4 or 8 ohms speakers? I thought that the lower load (i.e 4 ohm) can get more power than higher one (i.e 8 ohms.)

Any thoughts on this?

Thanks a lot,
Ake