I always get the impression from the steel cable it may contribute to that aggressive sound HD25 owners mention.
I've tried a few $400 cables, such as Tralucents pure silver, silver / gold. it will take some convincing from the HD25 sonics to go that far.
I think something around $50-$60 will suffice. :)
It will remain aggressive even after any recable. If you want to tame it, Akasa Paxmate Plus acoustic foam in the cup depths will do that and make HD25 sound a lot more like the Amperior as well.
A cable will just improve tone quality and bass quality for the most part. If crosstalk on the cable is lower than the stock one, you will notice the soundstage size increase a pinch. If you go with a hand-made cable, I recommend that you get one with a Viablue plug, or at least a Switchcraft one. The small difference a cable makes is made even smaller yet by low-end plugs in my experience.