My knowledge of tube amps is rather limited, which is why I'm asking this question. I read a FAQ on Line6's website about removing 2 of 4 power tubes for a lower output volume.

Q: Can I run the Spider Valve Mk II head with only the outer two power tubes (removing the inner pair) to operate at a lower output volume?

A: Yes, it is safe to run the Spider Valve Mk II Head with two 6L6s. It is recommended that the output impedance be run at "half" when running in this configuration. An example would be running the 8 ohm head output connected to the 16 ohm cabinet connection, or the 4 ohm output to an an 8 ohm cabinet connection. (Line 6 recommends re-biasing your amplifier after removing the tubes, as the current draw will have changed.)

My question is: Would this theoretically turn your 100-watt amp into a 50-watt amp? Therefore requiring less volume to drive the power tubes harder?

Also, why exactly do they recommend running the impedance at half?
Yes. This will turn your 100 watt amp into a 50 watt version.

It will also require less volume to drive the tubes harder, but understand in the big, big picture of wattage, 50 watts isn't all that much different than 100 watts. It's still loud.

You need to halve the impedance because you've removed half of the impedance being seen by the input of the output transformer. Removing 2 of the tubes changes the impedance at this transformer's input.
Ah, excellent. Thank you very much for the response, it was very helpful.