I am pretty certain the line fuse will blow before the transistors start to melt.
((I accidentally shorted a Forte' 4A channel once, and the spark was pretty scary! (The spark looked just like shorting a wall A/C wire) . But only the Forte's power rail fuse blew, and that was the only casualty.))
So i have confidence in Bryston's design, and the line fuses being able to keep the amps from blowing up. Since Bryston have a 20 year warranty, i am betting they are VERY confident in the design.
And I am certain many other amp designs do the same.