suppose the guitar player decides to turn up his amp past 11, causing it to explode and send a spike of energy back down the line.
FWIW, guitar amps don't explode. Typically they overheat the power transformer which melts the wax it's potted with producing smoke and an aroma that most guitar players know all too well. The other thing is that output tubes short out. One of two things usually happen. Either the 2-3 amp fuse blows or it takes out the plate resistor which creates smoke or the occasional fire along with blowing the fuse. Which is about as spectacular as it gets. Since there's a 2-3 amp fuse in the line side, there's no way any surge is going to be fed back anywhere. Everything is referenced to ground (or neutral on older non-grounded amps). A good modern amp should have flameproof resistors of sufficient capacity to take the fuse out.