Mods which don't harm re-sale

I'm considering getting some dynamat and dampening some gear, especially my CD transport. This leads to a general question: 

Which mods have you done to gear which -- when you went to re-sell it -- either did NOT harm it's resale value or even improved it? 

I'm not talking about different tubes, etc. but changes of caps, diodes, dampening, etc.

Re-sellers, what's your experience?
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.


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? 
Both my speakers sold at a premium precisely because I had modded and upgraded them. So the idea mods trash resale is unfounded. It all depends on the mod, and the buyer. There is a saying with Porsche that when buying used you don't buy the car, you buy the owner. Because experience over time has shown there are guys with the skills to make even a Porsche way better than new- and there are also guys inept enough to ruin even a new bone stock 911.  

By far the most cost-effective mod I know is better diodes. The very best diodes cost a lot less than the very best caps, but the improvement in clarity, liquidity and image focus and air is at least as great as with any cap at any price. Manufacturers (and everyone else it seems) apparently think that because the diode is the first stage in the power supply that it won't matter everything will be smoothed out by the filter caps. Whatever they think all you have to do is swap em out to know they make a huge difference. 

Diodes are not all the same, and just like caps you cannot tell by value or spec but only by ear. Sorry, just the way it is. Michael Spallone tried a lot and so I paid him to do mine on my Active Shielding and the difference was night and day.  

The thing about mods is not so much resale as what it does to your upgrade path. A few of the right mods and your component is so much better it is very hard to find another one without moving up a solid level in performance- which usually means cost. Essentially what happens is your $500 DAC sounds better than a lot of $5k DACs, so where do you go? 

The stuff you are talking, dynamat, doesn't even rise to the level of a mod. A much better one by the way is fO.q tape. Or Synergistic ECT. Stuff you can easily avoid this whole issue by simply removing before selling.
I’m a Mac guy worst thing you can do is a BAD job, best thing you can have DONE is a good job. That being said, when I see a Mac full of yellow caps it just turns me off. I see one rebuilt with quality PIO, and Teflon Russian surplus, I’ll give SOMEONE more for the build.. I look at what was done (the parts) and the quality of their work.. DON’T drill holes in anything.. Just silicone it in place. Especially a Mac.. Some Vintage Mac guys are pretty Picky.

They will even gut an old bumble bee cap and load it with a better aftermarket (yellow LOL) cap and reseal it.. Those are 2-5000.00 rebuilds too.. Time is money.. Mac guys are NUTS.. LOL

Upgraded Tonearms will usually net you dollar for dollar too, sometimes more..

I think it's buyer dependent. Myself, I never even look at modded equipment, others it depends on the mod and who performed it and others it doesn't matter that much. 
If the equipment is older that upgraded power supply capacitors should be a value added.  Most mods and be removed but overall IMO adding capacitors like copper V-caps, Audio Note resistors, upgrading a volume control should be a good thing.
Richard Modaferri’s mods to McIntosh tuners certainly increase resale value