I will not bother with vintage at all, you will end spending as much as a new amp, at the end, and you will have an old amp anyway, that could fail of anything else that was not replaced, even the wires.....just begin to pop BG caps or more expensive, and so and so, and you will end with a pretty high bill...unless you have them in stock of course...
Also you can get nowadays, choosing wisely and in the used market, for 300-500 very good amps, that will beat the dust of any vintage. I used to own, old Pioneers, Technics, Onkyo, Trio, Sansui, Kenwood, etc...amps from the golden era of the 70-80 in solid state, mainly because I love the sturdy construction, look, and sound, and I still do, but honestly my Marantz PM7200 sounds ages better than any of those I had, and mainly all of the them were class AB, not even Class A, not that this matters too much if the amp was well implemented, but another point down...
Now with the new generations of digital amps out there, the vintage will have a very rough time...Always look forward, not back, leave the past to the grandpas....