I know that there have been threads on this very subject, and I remember some one advising that if a component was coming out of a long period in mothballs it was good practice to use a "variac" (I am not sure about the spelling) between the component and the outlet when turning it on for the first time. I assume that this is like a rheostat of sorts. If I recall, it was suggested that introducing AC to a component that had been sitting for an extended period of time might be better done slowly. And I do not remember if this pertained more to tubed gear than solid state. I believe I remember a comment being made that caps do not like to sit for a long long time not being charged and discharging, as I think it was said that they "dry out" over time.
But it’s too late for that, and apparently you experienced no catastrophic results.
From my own experience: from ’08 to late in ’10 I used my system rarely if at all. In late ’10 I took it down to put a new floor in the LR which was, at the time, my listening room. Late in ’17 I took it out of mothballs and everything worked fine. I have since upgraded all the electronics except for the (tube) amp, but today I am still using that amp that sat dormant for over seven years. I remember the pre was noisy, but that was a bad tube or tubes and I replaced all of them.
I have another tube amp that had been sitting longer than my main amp, and in ’20 I fired it up when I was troubleshooting a signal cap on the main amp, and it did have a significant hum when I got it going.
I do not know enough about this stuff to say if the hum on the one amp came from sitting for a long time. As far as the bad signal cap in the main amp, I think I know what went wrong there and I do not think it had anything to do with sitting for all those years.
But if you did take your pre to a tech and get it checked and update, it seems to me as if that would be a selling point. At least I would see it as such if I was a buyer.
