I started another thread that presented the perspective of one electrical engineer that works in the audio industry doing analog circuit design here. That might be interesting reading on why power cords MIGHT make a audible difference in how your amplifier sounds.
Practically speaking, a no cost way to try this out if you are an Amazon Prime member might be to buy several power cords via Amazon. Plug them into your Hafler amp and see if you hear a difference. If you hear a difference, you've answered your question. Hear no difference? Power cords go straight back to Amazon. In my experience, any difference you might hear should be immediately evident OR perhaps power cords make no difference (or at least no difference on your amp).
I personally think a Shunyata Research Venom V14 power cord at about $150 is a great starting point based on listening to a number of power cords loaned to me by a friend, but I don't see that Amazon sells Shunyata products. Amazon does have Audioquest power cables & Pangea power cords available through Prime. For an amp, perhaps ideally find a power cord with greater current carrying capacity.