Mods which don't harm re-sale

This depends so many things I don't know if it can be answered. Same mod, one might say wow great added value, the next might say no way not touching that!  

So way too many variables to even hazard a guess. That's on the one side. On the other side, don't have to do too many mods to appreciate they are by far the most cost-effective way to get more sound quality. A couple $8 hexfreds made such a huge improvement in liquidity, detail and depth I could care less it would cost me hundreds if not thousands to upgrade to another amp that would get me that so I could care less what it's worth in resale. 

But then I am not a flipper. If you are a flipper then that is what you do, flip, and you'd be a fool to mod anything. But then flipping itself is foolish so it would be consistent and you probably should. It all depends on how you look at it. 

Only if changes or refurbishment were performed at the factory would be easier for a buyer to digest.
Otherwise resale value will drop whatever the mod, successful or not.
Tweaking that can be removed is different.

G

This will kill the resale do not do it unless you like to loose money.

Good answers. I'm not a flipper, but I know that after a time, one might want a new and better piece of gear and that at least not *tanking* the resale value makes sense. Maybe there are not that many dollars involved, overall, but I have friends who swear by cap changes (to DACs or to amps) and that is another zone of mod which inspired this question.

@ebm  "This" will. What is "this"? A number of different mods have been mentioned. If you have advice, can you be specific and possibly explain?