Surely this fear of the Magma GM100 tubes blowing (joking aside) is an urban legend?
The infamous Ken Kessler exploding Transmitters tube episode was an entirely self-inflicted problem, indeed Nat were pretty pi$$ed off as to how this unreasonably reflected on them.
Afaik NAT had some QC issues around the time they became really populair. This seemed limited to DOA or not fully correctly functioning units which needed to be returned. That alongside of lengthy delivery times gave them a bit of a bad rep in these parts of the world. Seems to be fully resolved now though, they continuously seem to improve both looks as build quality aswell. Attached photo is of a recently delivered transmitter.