The trouble with an essentially complete rebuild, like the JBL SA-600
@amirm just reviewed, is no matter how carefully it was done, is the amplifier is likely to perform quite differently to the way it did when new.
As such, you have no reference to compare. Even if you did, that reference would be over 50 years old in the first place and likely deteriorated too.
I'm dealing with such an issue with these classic Marantz 1060s on my bench right now:
View attachment 228095
No, you're not seeing double!
The top one is my father's 1060 bought in 1972 and kept boxed for 40 years. I fully restored it about 5 years ago as it had deteriorated in the box (caps of course), but didn't change any semis as they were all in spec and silent (no noise). I borrowed it yesterday for the purpose of comparison, as I have all the data and know exactly what I did and how it tested. All the FR plots and performance details are in my records for that amp.
The bottom one is a friend's 1060, one he bought on my recommendation which he thought was likely original. It wasn't. It had been completely (and very well done) rebuilt including all the front end (phono/pre/tone) semis, caps etc replaced. PSU and amp caps as well. But It sounds like #ss to him.
Now I'm delving into what exactly is causing these two essentially identical amps to sound totally different.
More to come...