IMHO, making vintage gear reliable involves a lot more than coupling caps and tubes.
Much vintage tube gear contains carbon comp resistors which are hygroscopically reactive, and when they go out of spec, rather counterintuitively, they increase in resistance. So all carbon comp resistors should be checked. Sometimes, they can be oven baked to get them back in range.
The next problem is usually line voltage, which at my place runs around 122V. Most vintage gear is designed to run at 115-117V, so this should be addressed if ones line voltage is high. A bucking transformer, variac, or even a couple of thermistors in series should knock down the voltage.
And if the vintage tube gear is stereo, a fair amount of it will have PECs in it. They can last forever, or they can go tomorrow. I replace most of them with discrete circuits, YMMV.
And common selenium, or "top hat" diodes, suck and release toxic gas if they give up the ghost, so they should be replaced with modern diodes. The problem here is that modern diodes are more efficient and the downstream voltage will rise, necessitating a dropping resistor.
And back in the day, tubes were cheap, so much vintage gear runs the output tubes hard. A problem that is exacerbated by higher line voltages. Especially if using new production output tubes, bias voltage will need to be adjusted, some vintage gear does not allow bias adjustment, so a work around needs to be installed.
