I'm really into vintage audio equipment. Where/if in Tokyo might one find second-hand, high-end, Sony, Denon, Technics, Pioneer, Nakamichi stereo components of 70s/80s/90s vintage in excellent condition?
While this is likely to be off topic, here's an answer:
This is THE spot for buying vintage electronics.
While it seems from the outside that Akihabara is only anime and manga everywhere, originally the area was (and still is) the best spot for electronics.
You want to visit the maze of small shops under the tracks of the Sobu main line. This image shows where you can find the many entrances.
Keep in mind that there are many other shops around Akihabara, it would be too long to catalog them all, but I encourage you to explore around.
While Akihabara is the "place to be" for this kind of stuff, there are other places for this. One good example is Nakano Broadway. You also might find what you are looking for in stores like hard off and book off, which you can find about everywhere in Tokyo (and Japan). Also, roaming around Shinjuku, you will find plenty of vintage electronics shops.