If the engineer can't figure out what he is looking at, you have the wrong engineer. The circuit is straight forward tube design, each tube is independent but has a common power suppy. You likely have a screen resistor blown from a crap tube, that is about the only thing that can go wrong.
VTL MB450 Schematic
I have a pair of VTL MB450 signature monoblocks. Has anybody got a copy of the circuit schematic ? I have been trying to contact VTL but getting no response. From reading some of the threads this is not unusual ! One of the amps is in for repair and the schematic would assist the engineer.