Vintage amps & power cords

Roxy, can't imagine the value being negatively impacted as long as the work is good. I'd pay more for the improvements you've had carried out.  

noromance,
Of course your attitude makes sense, but that isn't the way many people see it. Their admiration for a certain brand or specific component becomes perverted into a fear of making any change to it whatsoever lest it no longer be "original". These things are just electronics in my view, and if they can be improved or even just maintained to perform to their original specs, why not?
I think it's funny when I see a for sale ad here on Audiogon that describes the component as "unmolested", as if changing it is akin to rape. Some even brag about it never having been opened or serviced...after 20 or 30 years.

My intent was not to begin a debate about the virtues of aftermarket power cords. Personally, I know they do contribute to sound quality, especially in high current/voltage/power solid state power amplifiers. Impact on resale value aside, I would think safety should be a concern as well if the cord is no longer pliable. I also have a CJ Sonograph amp that could benefit from a (power cord) upgrade.

I've had my vintage gear changed to remove captive cord, and the results have always been positive. The safety concern on a cheap 40 year old power cord is very real.

Any gear owned by an audiophile who cares about his or her credibility among other audio geeks (non geeks really don't care at all) should have the thickest possible AC cable, and that cable should be visible so you can point it out. I have a water hose thickness cable to one of my subs that is completely hidden under stuff, and thus is kind of wasted…it doesn't "sound" any better than a more pedestrian AC cable…but still…this sort of thing is important.