I know the pic you're talking about, no doubt there is an added preamp tube, a master volume and KT88s. The extra preamp tube is actually for an effects loop, that's what the additional jacks on the back of the chassis were for. That amp was actually a Super Bass too. The problem with it though is Page's Zeppelin heads were US export amps with the polarity selector on the front which is what the 3rd switch was for and no voltage selector on the back. The pic you're referring to has a voltage selector on the back which would point to it being a European export, only 2 switches on the front.