NAD 4020A issue

Sounds like a failing solder joint in a high current connection. As the metal heats up, expansion takes care of the resistance. A temporary scenario. Failure is almost guaranteed as the solder joint weakens further.

NAD and other lower priced products are famous for poor soldering at the factory (these people were probably farmers the week before they got hired) and so they were not savvy as to heat vs. conductor material, size, etc. vs. flux type that more high end brands were (sometimes, alas, not always) able to train their manufacturing crew.
OF course, some high end is no better made than low end, i.e. the soldering and PCB materials that Nakamichi used. Good sound, junk build...flux everywhere, phenolic boards, junk...

Even good brands have solder fail, it is a lead and tin alloy, after all.

Repaired more NAD, Proton, Sansui, vintage McIntosh SS, etc by reworking solder joints than ANY OTHER method of service by far.

That's good to hear, Mechans. I was hoping to replace the problem part; it makes a awful racket through the tube amp when trying to power up. I always power up the tuner first, then the rest of the system...others family members often forget to do this. :P

It sounds like a simple powering up procedure and if I were you I wouldn't worry.