Install IEC male socket or not??

Hi. I own a Threshold Forte Model 3, which is a superb solid-state amp. It is a bit older though, and it still has its original slender power cord and two-prong plug. This is a 200 wpc (8 ohms) amp with high bias and high current, and it just seems to me that I ought to install a male IEC socket on the rear panel and buy a proper power cord. I do worry about ruining the resale value though, and I've even heard these old Nelson Pass amps have some modest collector value. I welcome all learned opinions!!
If I had that amp I would want to do it, but I would contact Jon Soderberg first to get his thoughts. It won't hurt to try if you already haven't - or, contact Pass, they might do this for you. As long as Jon or Pass provides the upgrade, I don't think you should worry about a diminishing resale value... JMO.
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.
Bdgregory states it correctly. I have done IEC's in a number of components, very worthwhile from a sonic standpoint.
I agree with the above posts in that you'll appreciate an iec for a better aftermarket power cord. Those 200 watts will appreciate a larger guage cord and your ears will appreciate the improved sound as well.
I installed one on my Hafler dh500 using a dremell tool w the right bit to cut heavy gauge metal. Took about 15 minutes, looked perfect, much more comfortable to work with, n changing power cables made the sound improve. Cant think how it could affect resale value imo.
I've done this on many pieces of equipment, but would like to find the "appropriate" punch ? to do the job more effectively without the tedious bullshit, or fear of ruining.

I looked into Greenlee from McMaster-Carr, but they don't seem to have the exact punch shape for an IEC cut-out (D punch)

What do companies or modifiers use, where do they get them ?
(that's the million dollar question)
Yes, a punch would make things easier, however, you would need some kind of backing for a punch. Without backing and/or reinforcement behind the panel you're not going to punch out most of these panels. In most cases I think we're stuck using drills and dremel tools.
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.
The only alternative I came up with would be to take an IEC inlet and trace it (scribe) to a small, square piece of material, cut it out to perfect size, then use it as an overlay-template guide.

Surely, a company that does mods must use some modern method (IEC punch) other than nibbling or a dremel tool.

I'll call a few places tomorrow and ask.
Yes there is an IEC chassis punch available from Greenlee; I don't know where but I saw one in a catalog somewhere...

Actually there is a simple method to modify for testing upgrade cords without butchering the back panel, and at nominal cost. If you like the results then that can be done later... If not then you can easily go back to stock. I have previously posted this many times; here it is again:

Start by getting a male-prong IEC (which you may end up installing later - or not) there's many sources for this.
Cut the stock cord within about 6" of the component. Strip the ends and solder on your IEC. Use heat shrink sleeving, or at very least electrical tape, to insulate the bare connections.

If you want to return to stock cord, then just strip both the ends and solder-splice together again (or use butt-crimp connectors). Again use heat shrink sleeving for a professional appearance. You can heat the sleeving with the soldering iron, or a hot hair dryer, or even a butane lighter if you're careful.
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.