Is this on the amp model or the real amp? On the amp model, that should be unnecessary (and there isn't a straight forward mechanism in the Axe that would do it that I'm aware of) because you would simply turn the master volume up to get more power tube distortion and the output level down to avoid clipping.
The typical use case for pulling a tube pair in a 100W+ amp is to reduce power to get power tube distortion at a more tinnitus-friendly volume level. There are some subtle changes in feel and tone, but in my experience they're pretty small, and on an amp like the 5150 that derives so much of its tone from the ridiculous amounts of preamp gain, I wouldn't think there would be a huge difference.
For the 5150 combo that was designed around a 60 watt power section, odds are that the tonal differences between that and its 120 watt big brother will be more from the smaller power and output transformers than the two less tubes themselves (unless, of course, Peavey for some reason kept the bigger transformers from the 120 watt version, which wouldn't make sense from a cost control perspective).
All that said, fiddle with the Power Amp section's transformer settings, or perhaps do a tone-match of the actual combo if you have it?
