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?