I also had the 1/2 power option added (which answers one of your questions).
I'm pretty sure you are referring to a Pentode to Triode switch. Not the same thing as a 1/2 power switch. A 1/2 power switch in an 18 watter would require going from push-pull to single ended output. That would be nuts.
I didn't build the amp, I'm not an amp builder, all I'm saying is that Nik added a switch to the back my amp that (to the best of my knowledge) cuts output power in half... it very well could be a pentode-to-triode, I'm not sure.