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.
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.