My JTM610 combo isn't the most rugged combo amp I've owned but then again, in my experience at least, tube amps in general aren't very rugged pieces of equipment. Every tube amp I've had has had some sort of problem at one time or another! But nothing compares to the sound...
Overheating seems to be the problem my 3 speaker combo has while the cabinet itself isn't the sturdiest design. I've learned how it needs to be treated: A long warm-up time before play and plenty of ventilation.
Award for strurdiest cabinetry of a combo goes to my old Ampeg which used wood a couple inches thick. The problem there though was very weird tubes that were impossible to replace and nowhere near the majesty of that Marshall crunch.
Brother musician listen to a miracle!