Install IEC male socket or not??

I've installed IEC connectors in multiple vintage amps, and some of them I've sold with no concern from sellers or apparent impact to value (ie anything it was a benefit). I was completely open about the fact I added them and that I did it myself. Frankly my first one had some issues of workmanship (a gap) and the buyer was perfectly happy with it.

If you are of a mind to do it yourself, just be aware it's very hard without a die/punch to get the hole cut out perfectly. A dremel tool really doesn't work on heavy gauge steel. I end up using a combination of drill bits, nibbler, hand file, and dremel grinder. It's tedious.

I really wish I could find a punch for the exact hole size, but I haven't found one yet.

Also, check inside your amp. Some of them are a real bear to properly place the receptacle without interfering with other components. An alternative is to install one of the cord end IEC connectors on a pig tail, but I thing those look silly and am not sure what they accomplish over simply installing a good quality captive cord.

I've done this on many pieces of equipment, but would like to find the "appropriate" punch ?

I would too. I have a punch set I bought at Harbor Freight - but it only does round holes (you see these on ebay too). The set cost $15 and it includes 4 different size dies. The other problem is the smallest die is too large for an IEC, otherwise I could start there and nibble away the remainder. I searched the internet for an IEC shaped die thinking someone would make one. The closest I came was a rectangular die for $500. Surely someone can do better on that price.

a slightly more attractive alternative is to get one of these. I have used these in a few situations but don't like the pig tale dangling. They can be as effective as a flush mount IEC though, and super easy to install.
Parts Connexion use to stock them but I couldn't find a good quality on their site this am.