One issue on the solid state side is the availability of replacement transistors. Some very good amps used transistors that are becoming or have become unobtainable. However, even with that, if the amp is that good, you could always look for a used/broken one to part out.
The other thing I think about is the precariousness of the "one guy shop" that a lot of our great sounding equipment comes from. Since there is often no succession planning, if something happens to the "one guy" the company may cease to exist. The gang of folks that got into the business of designing/manufacturing high end audio in the 70's, 80's and earlier is starting to get pretty long in the tooth, so when purchasing older amps a consideration is whether there will be anyone around to repair it if needed. Since most of my equipment comes from "one guy" shops, I can only hope that there will be competent technicians available to repair the equipment as long as parts are available. Of course, the other option is to make sure the "one guy" is younger than you are :>}
The other thing I think about is the precariousness of the "one guy shop" that a lot of our great sounding equipment comes from. Since there is often no succession planning, if something happens to the "one guy" the company may cease to exist. The gang of folks that got into the business of designing/manufacturing high end audio in the 70's, 80's and earlier is starting to get pretty long in the tooth, so when purchasing older amps a consideration is whether there will be anyone around to repair it if needed. Since most of my equipment comes from "one guy" shops, I can only hope that there will be competent technicians available to repair the equipment as long as parts are available. Of course, the other option is to make sure the "one guy" is younger than you are :>}