Mods which don't harm re-sale

Footers are one way. Its more a tweak than mod. But  It's amazing that even the material, (wood, vs metal vs stone) can make a difference. My 1st encounter was reading about pumice stone under my cdp. Being a skeptic, I figured I could afford a couple bucks. My jaw dropped. Night & day? No. But heard more spaciousness. I moved on from there to roller balls. My wife was surprised that she could hear the difference between the material the balls of which were made. I'd think the dynamat was an acceptable mod. 

I've owned a number of Klipsch speakers and have replaced crossovers and drivers as well as added Dynamat around the horns.  I don't think it necessarily increased their value, or that I got what I put into the mods back in terms of resale value, and based on some of the responses I received when I tried to sell them, I do think it added to their appeal.  And I got to enjoy the results of the mods while I had them.

For me, if the owner makes mods to the electronics, that's a turn-off.  If they have the manufacturer or a qualified service shop do it, then it's less of an issue, but may still be a turn-off.  Is it really an "upgrade" or a change from what the designer intended?

I recently bought some Herron amps that had been refreshed by Keith.  That definitely made them more appealing.  

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