I just now stuck an edit in there about my pre being noisy when I got it running again due to bad tubes, but that would not have been related to sitting.
Your question would be better answered by someone like @atmasphere ,as all I know how to do is make basic checks with basic meters (like a cap checker) and desolder and solder. I was told that the cap checker I have will not predict the life span remaining on a cap, and even if a cap checked good with my cap checker it didn’t necessarily mean it is a healthy cap. Only that if it checked bad, it was definitely bad. I do not know if a tech would have equipment that was sophisticated enough to verify that.
I’d say that if it is a real nice preamp and on top of that you had the film caps updated that would make it more of an attractive buy for some one. But I don’t buy and sell a lot (as a matter of fact, i don’t sell at all) so I do not know. Maybe get an estimate on that and see what it would run?
I started a thread, myself, inquiring about the life of the large electrolytic caps, and I seem to remember getting the idea that they held up better than film caps and I was okay. I was going to change them in my amp anyway, just because if one let go I felt it would be a nasty mess and they would be a breeze to replace in that amp being as there wouldn’t even be any soldering with screw in terminals, but that was a few years ago and i never got around to it and there have been no issues. That is a 23 year old amp and with the exception of one signal cap, there have been no cap failures.
EDIT: OOPS! I read your last post real quickly and I thought it was directed my way, and just now noted that it wasn't.