Replacing Captured Cords with After Market Cords?

If you don't want to drill the box, just cut the cord, but leave enough room to work with, buy an IEC, install on cord, heat shrink it. Walla.

Make sure you identify which wires are hot and which neutral right after you make the cut.

Two possibilities are illustrated on mapleshaderecords.com
First choice is to plug your fixed power cord into the top of the line Mapleshade powerstrip, and you will think you upgraded your fixed power cord. At least, that's what I found to my surprise, with my venerable fixed cord components, such as Cary SLA-70Sig amp.
Second choice, is to purchase top of the line fixed power cord upgrade kit from Mapleshade, where you clip off most of the original fixed power cord, using the remaining power cord pigtail as the start of your new power cord upgrade which clamps onto the old remaining pigtail, according to Mapleshade.
I simply used their powerstrip to get better transient speed, clarity, and increase in dynamics, rather than do power cord surgery on my amp.
Mapleshade is very responsive to email questions, so don't hesitate to check with the manufacturer.

I think the idea here is to eliminate any degradation of power which may be contributed by using the stock cord. If you put ANY device or modification before your stock cord,your stock cord will still degrade whatever is being fed to it. An add on is definetly not the way to go here. IMO