The combo is switchable to 50 but the head version isn't.
And there are some differences in the 50 watt head, there is a video of it on youtube where a dude from Peavey talks about it. There is also a another thread here that shows all the specs.
EDIT: Here is the info from the other thread
“Traditionally, 50-watt amps always seem to have the perfect amount of volume and sag to create a true rock & roll moment,” Satriani says. “But the golden era of the 50-watt didn’t see a version that had a variety of sounds or were actually useful tools for the gigging musician. Peavey and I were finally able to bring this idea of a really rockin’ 50-watt into the modern era where real guitar players can use it. We wound up with a really bold, in-yourface sound.” - Joe Satriani
• Two independent channels, Clean and Crunch
• 2 - 6550 power amp tubes
• 5 - 12AX7 preamp tubes
• Global Presence and Master Volume controls
• Global Master Volume Boost with switch and level control
• Six-way attack control on Crunch channel
• Pre-gain Boost switch on Crunch channel
• Built-in MSDI microphone-simulated direct XLR output
• Line out with level control
• Active effects loop with send and return level control
• Rear-panel impedance selector
• Tube bias adjustments on back panel
• Four-button footswitch included