Typically, the most reliable amplifiers are class AB or D SS devices, which are kept ON all the time and which has UNREGULATED big iron SIMPLE power supplies. The reasons are:
- Heat is the number one enemy of any electronics. Class AB and/or class D generates very little heat when there is no input. Minimal quiescent bias currents. Valve amps lose out in this area.
- Keeping it on continuously ensures that the amp does not experience inrush currents, which MAY reduce the reliability of power supply capacitors.
- Unregulated simple power supplies are the MOST reliable power supplies. When SMPS comes into play, reliability will be less.
All the above are more easily achievable in an SS amplifier. So, all things being equal, SS amplifiers are likely to be more reliable.