What silver faced years should I avoid, or conversely, look at.
I'm kinda partial to the black face cosmetics, but if I can get the same sonic performance out of a silver face and save a few bucks, I'm all for that.
When it comes to vintage audio gear, the silver-faced era generally spans the late '60s to the early '80s. The transition from black face to silver face coincided with some circuit changes. Some audiophiles argue that the earlier black face models have a warmer sound due to different components used. If you prefer the black face cosmetics, consider looking for models from the late '60s to early '70s. They often have a distinctive sound that some enthusiasts appreciate.
I've owned several over the last 40 years and I don't think there are any years other to avoid although the final models with the pull-boost are not as desirable as the earlier models in most people's opinions. As I'm sure you know, components in all vintage amps drift/change/fail over time, so individual amps vary considerably depending on how hard they've been used, where they've been used, etc., so I'd always want to play a vintage amp before making a decent investment.