well im no veteran, but i do have something to add. in general, i agree with the general consensus of this thread. i.e:
1. high quality files (i do believe 320kbps mp3s are sufficient)
2. headphone/ mods and pad changes
a cable is meant to allow the signal through from device A's output into device B's input. that is all. the ability of the cable to do this depends on its build quality and impedance. the latter is directly relative to the thickness of the wire. what im trying to say is that a bad cable can degrade music, but a good cable can not improve it.HOWEVER, because of what i have written above, i never felt any need to invest in high end cables of any sort. so all that i said above is not coming from personal experience. hence, i may well be wrong. but in theory, i dont think i am. at any rate, the importance and the effect of cables is a dangerous topic here, almost as flammable as the old lossless vs. lossy debate. its best to read up about these subjects as much as you can, and then decide for yourself whats true and whats not. taking anyones word for it, mine or anyone elses, is imho unrecommended.
one more thing to consider is the setup as a whole. assuming your source has a good quality dac and amp of its own, and your using low impedance/high sensitivity headphones, you may very well not need an amp at all, and adding one would be superfluous. on the other hand, assuming again that your source is giving a high quality signal, but your using high impedance/low sensitivity headphones, then no matter how good the headphones are, and no matter how good the signal provided by the source is, the music wont sound right without an amp (external or otherwise) powerful enough to meet the electrical requirements of your "hard to drive" headphones. so it all depends.
it also depends on your preferences. what you deem as high quality isnt what i would. some like tube amps, some like solid state. some people are looking for bass emphasis or treble happy sound, when others want linear, neutral sound.
in any case, its important to look at the whole picture, and not just evaluate each individual component. you could decide that the headphone is the most important bit and therefore pay 1.5k for a pair of HD 800s. but running them off a fiio e6 just wont do. see what i mean?