I'll try to elaborate a little, as I see now that while trying to be brief (I was typing it out on my phone), I think I probably succeeded better at coming across as somewhat arrogant than to explain my reasoning - which was not my intent at all.
So - if a power cord is sufficiently dimensioned to handle the maximum current drawn by the device powered by it, it is good.
In the case of Norwegian rules, 16A mandates 2.5 square millimeters (Approx. AWG13) in permanent installations. This cross section is deemed sufficient to conduct 16A continously without leading to excessive heating of the conductors.
Power cords would normally be somewhat less generously dimensioned, as they are not required to carry the rated current (16A in this case) contionusly - so the cord feeding my power amp (rated at 2*55W into 8ohms) is 3*1,5sqmm (AWG15.5). This would translate into an ohmic resistance of some .02ohms or so for 1.5m/5ft of cord - in turn causing a voltage drop of some 1/3 of a volt or so when the cord is run flat out (16A) as compared to the no-load voltage.
This voltage fluctuation should pose no problem at all to a power supply.
(Not that the amp is capable of drawing more than a fraction of this current - but anyway, the cord is sufficiently dimensioned.)
I don't know the capacitance of the wire, but at 50Hz it is a safe bet that the contributed reactance is negligible.
Those two factors aside - voltage drop because of inadequately sized wiring/heat generation because of cord impedance - I really cannot see any other challenges for a power cord.
If loads of (electrical) noise is piggybacking on the desired mains supply, it is easily handled by the power supply; if not, a mains filter might be advisable; if I were to buy one, I suspect going to a reputable dealer like RS Components, Farnell or similar would provide better bang/buck than a hi-fi dealer.
Anyway - if I were to determine whether I needed any kind of power conditioner/mains filter, it would be most useful to look at the DC output from the amp's power supply; if there are no noise transients present, any noise on the AC side probably wouldn't affect the sound. This is what power supplies do.
So, to wrap it up - I am quite convinced that if swapping power cords causes audible effects, further investigation will show that one of the cords (at least) were not up to code.