A client brought in his very expensive Hi-Fi speakers for repair. The damage was fairly extensive, and we had discussions with him regarding the other devices in his system. He said that he'd already had the amplifier serviced elsewhere (not realising we had an electronics repair service as well) and that the amp had faulted, and this is what had caused the damage to the speakers. We repaired his speakers, and he was entirely satisfied.
Six months later, the same client was back with his speakers in a similar condition to the first time. This time, we suggested he bring in his amp and pre-amp as well. Even though neither unit had faulted, we knew something had to be wrong for this damage to occur again.
The amp and pre-amp were of the same brand, and he had purchased the entire system at once as a "matched system". We inspected both units and found evidence of the previous amp repair. Although non-original parts had been used, they were of the correct values, and the work had been done well. Neither unit, when inspected individually, had a problem.
However, when operated together, our analysis showed that setting the pre-amp volume control past one o'clock produced a distorted 'square-wave' output signal. This is what had caused his power-amp to fail in the first place and was also the cause of both occasions of speaker component damage. For this particular pre-amp, the 'full' volume setting was actually at halfway - allowing for varying volume levels of program material without distortion.
The client went away with a fully functional sound system, once again, but this time he also knew exactly where his maximum settings were. It was well over a year before we heard from him again and that was a phone call to say that the system was functioning perfectly, sounding great and when he wanted it loud he got it louder and cleaner than ever before - with no damage.