reports of the MRX*20 series (or even the previous version) of it not powering up.

Take out the mains lead and continually press the Main on/off switch several times until the blue light stops trying to come on and then insert the mains lead and try again.

What could be happening is that the processor gets a stray or ‘confused’ message from a control system and locks up which resetting the mains should cure, however these amps store a fair amount of charge in their capacitors and takes a while to “drain out”.