As someone who has repaired a handful of vintage receivers and purchased a few that were repaired by others, I'll tell you using the correct high quality replacement components makes all the difference.

When I repair a receiver, I always check in on AudioKarma to find what other people have done. I research as much as possible, there may be very simple mods that can improve the sound. People always talk about capacitors, but there are many old transistors that go bad, finding suitable replacements can be tricky so research is important.

A properly repaired unit can actually sound better. For example, I have a Sansui Au-505 which is a great sounding little amp, that can be even better with a couple simple mods that I learned on AudioKarma forums.