Are aftermarket power cords required to be UL / CE approved?

The National Electrical Code requires that all items installed in a building be tested by an NRTL, generally that means UL listed. This does not apply to your personal use items, but does apply to any fixed appliances or electrical equipment that is installed within your home or commercial facility. If your local jurisdiction has adopted the national electrical code then this requirement applies.

@rbstehno 
Sorry, I was in the property insurance claims industry for almost 40 years. Although HO policies do have certain exclusions, there is no exclusion for stupidity or carelessness. The average person hiring an electrician would not be well versed in the various wire gauges and would claim ignorance. The claim is paid and the insurance company, hires a cause and origin investigator, documents the cause of the fire and Ins. Co. subrogates against the electrician. The wood stove example is just an example of the homeowner's stupidity, claim is paid and there is to responsible party to subrogate against. All the insured has to say is "gee I didn't know". Of course, permits and inspections as required by local jurisdictions should always be obtained for the homeowner's peace of mind.   

Your stove example would qualify as an installed appliance, so yes it would be required by code.  If you hire an electrician to do work in your house, the parts he uses would become part of the structure and also need to be UL listed.  Anything you plug into a wall socket is a personal use item and there is no requirement that it be UL.

Do you call your insurance agent for audio advice?

doltwithlife has asked an excellent question. I'm surprised that nobody has suggested that he simply write an email to the manufacturer to ask.

I am considering getting an aftermarket cord for my Krell amp and until now I never considered this issue. I don't have a single 120V appliance in my house that uses as much power continuously as my Krell (I don't use space heaters). Plus, the standard cord is pretty long. Now that I think about it putting a cord made in somebody's garage on this amp is probably not a good idea. Before I buy an several hundred dollar cord I am going to check to see if it is UL listed.

One more thing. I think the insurance angle is a complete red herring. Whether the insurance fights your claim or not, you still had a house fire. I burned up the kitchen once through sheer inattention and stupidity and trust me, even though the insurance paid the entire bill, it was very scary and a huge hassle.