Here is the thing. The transformer's Scott uses can probably handle anything you throw at it. The half power switch is great as a feature and for those that want to casually mess around with it - and you probably wouldn't hurt anything (at bedroom levels).
But the Splawn User Manual specifically says to cut the ohms in half while in half power.
Now, if you plan to gig out like that or keep it that way long term, that is where Scott is saying pull the outer pair of tubes because you are not using them. There is no signal being passed through them. But they stay lit and the heaters are still active so you might as well take them out. Now, by doing that, you need to cut the ohms in half. Then, lastly, you should also go ahead and rebias.
I hope that clears that up as far as the difference. Short spurts, you don't have to do anything. Long term use, cut ohms and rebias.
Now on the rebias thing, I personally follow the advice of not biasing to numbers. Bias to tone. There are too many variables. Mainly, the plate voltage coming off and the physical variables within the tubes themselves. If you've seen my biasing tutorial you will see that I go into this in pretty good detail. That said, I have been running strictly in 1/2 power mode for a very long time now and my current set of EL34s are running at 35 mA right now.
So - long answer
there is no set rhyme or reason on what your bias should be at half power. Generally, my opinion is that it is not going to change all that much.
