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.
