I bought my Quad 303 new in 1978. It was still sounding great when I had it serviced by Quad in 2021. My Sumo Polaris II was untouched and working beautifully when I sold it in 2023, at 28 years old. I bought a recapped Mark Levinson 331 in 2020 - zero problems, and sold it for a profit last year. My star SS amps are my Michell Alecto II monoblocs, serviced by their designer in 2019 and still sounding fantastic at 30.
So I’ve had precisely zero problems with old SS amps over many years. As base class A/AB technology is pretty mature, they’re a good buy, and are a good way to experience different design philosophies and circuit topologies.