It isn't that they aren't reliable, they very much are. It is when a tube fails and takes out other parts inside the amp. Most manufacturers use a fuse for the tubes but ARC chooses not to for a purer signal. If another manufacturers tube amp fails, it is usually replacing the bad tube and fuse. With the ARC, it is usually a trip back to the factory. That Ref 75SE is quite a special sounding amp, I have heard it driving Wilson speakers and Revel speakers. In both cases the sound was excellent.
