Replace pwr cord w/IEC socket on vintage gear?

You don't need to put an IEC inlet on the amp. You can just remove the existing cord and replace it with a new power cord. It will make experimenting with different power cords more difficult, but it will require no drilling or cutting out holes in the chasis.

I don't think the amp was voiced for the stock power cord. The amp probably wasn't voiced at all. It was spec'd out, and well spec'd out at that. Heathkit offered some excellent products. It's fine to replace the power cord, particularly if it's frayed or totally wimpy, but swapping power cords to find the right one kinda' goes against the vintage vibe, if you know what I mean. Just stick an Acme or Volex on it -- it's more along the lines of what the designers at Heathkit would do.

Rrog, you really think the engineers at Heathkit listened to the amp with different power cords and "voiced" it to their sonic preferences? You need to be careful when applying modern sensibilities to times gone by.