.... even if drilling holes on the top part of the chassis shouldn't really hurt the amp 
That very information is particulary interesting!
The tubes were NOT replaced (which means the tech saw no reason to replace them). I don’t recommend that you replace them unless there is a reason. Preamp tube selection is not always as simple as just slapping new ones in there - an “old” one can be great, and a “new” one can be noisy. You can try it if you want, but I would mark, and keep, the old ones.
That very information is particulary interesting!

I left the band business and now only play at church or as a fill-in here and there and that's mostly in-ear / no amp. I might end up selling it.