This is, in my opinion, the greatest weakness of modern, complex, mega-mode amplifiers: they're difficult to repair, and have drastically reduced life spans. To understand this, consider the difference between aircraft and automobiles:
Visit any local small airport, and you are likely to find this year's latest offerings hangered side by side with pristine models from the 1950s, 1940s, and even earlier. This is because aircraft are designed for maintainability, whereas automobiles are designed for obsolescence. Every component of an aircraft, right down to its airframe and covering, are replaceable. If you purchase a 1940 Stinson Voyager with a hundred hours since major overhaul, you are assured of having a vehicle every bit as safe and serviceable as a more recent Cessna or Piper with similar maintenance currency. Conversely, for all practical purposes, once the body or frame of your otherwise well-maintained automobile rusts out, it has effectively reached its end of life.
In the guitar-amp domain, the reality is that a PC board can only be de-soldered and re-soldered so many times before the traces delaminate. That point is the beginning of the end for that unit. A skilled tech may be able to hand-wire a jumper here and there, but ultimately, all you can do is rescue the chassis, transformers, and a few components, and build a new amp.
But there's even more to it than that.
Members of my generation may remember tinkering with their own cars, either in a quest for more performance, or simply as an economical approach to keeping their vehicle running. Through the 1971 model years, engines were pretty basic, and could be tuned, tweaked, and even rebuilt by the average backyard mechanic with no more specialized tools than a tray of wrenches, a timing light, and a set of feeler gauges. To maintain today's vehicles requires computerized analytical machines that cost thousands of dollars. Moreover, most components can't even be economically repaired. You simply replace an entire (usually expensive) module.
I design and build my amplifiers with three criteria:
The first two are subjective, and since they represent the goal (or purported strength) of virtually all such products, largely moot.
The last is an aspect I share with other so-called 'boutique' amp builders.
I've read posts in various amp forums expressing reservations about buying products from small builders based on future repair needs. In fact, this is far less an issue than it is with most mass-manufactured units. With the exception of a few giant egos who feel their ideas are so innovative that they encapsulate them in epoxy blocks, a properly-documented, cleanly-constructed, hand-wired amplifier will be easily serviced by any competent technician, at a far lower cost than any PC-board unit.