my experience:
bought a vintage McIntosh Tube Tuner/Preamp mx110z made in 1964. Took bottom off, no repairs or alterations visible.
Played, sounded great.
What the heck, dropped it off at Audio Classics, they replaced 10 capacitors, and 15 resistors. Nothing 'blown' or 'dead', but they get 'out of spec' i.e. no longer +/_ 2%, old stuff +/- 5%
More importantly, they replaced all the old corroded jacks with new gold plated ones.
Sound better? I don't hear any improvement, but those new jacks give much more secure connections. Before, I was having intermittent imbalance, jiggled a cable, ok, next day ,,,