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?
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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