If you pull a pair of power tubes (inside pair or outside pair, doesn't matter which), drop the impedance switch by one setting. Mike and Bill told me once in the past that with half the power tubes running, there's half the resistance happening in the power section, and dropping the impedance in half compensates for it. Also, the bias may or may not have to be adjusted if you're going to run the amp like this for the long term to help the tubes live longer. So set the amp to 8 ohms if using a 16 ohm cab, or 4 if using an 8, if you're going to run at 50w.
Side note: you can do your own Haynes mod in 3 seconds with a soldering iron. De-solder and lift one leg of the treble bypass cap on the lead channel's gain pot. I run both of my SLOs like this. To my ears, it brings down the "chirp" and allows a little more wiggle room on the treble and presence knobs. I usually run the gain at 4-6.