In an expensive home stereo system (let's call that more than $3,000 for source, RCA cables + power cord, amp, speaker cables, speakers), you will easily notice a difference especially with interconnects between a $20 cable and a $150 cable.
I don't know if the same can be said of the cable from the output of a DAP to headphone cans. I assume so. If you listen straight from
- mp3 128 in DAP ->headphones, probably no difference between stock cable and custom cable.
- mp3 V0 or mp3 320 in DAP->portable amp->phones, maybe a little difference.
- FLAC or other lossless to portable amp-> phones, probably a little more noticable.
- cdp->plugged in home HP amp-> phones, then the difference is probably very noticable.
So it really depends on the quality of the data, the source, the amplification, and the phones themselves more than the cable into the phones.
I like KRMATHIS' rule of 10%, except that in my home stereo I have about 20% of my budget in the connections (ICs, power and speaker).
But the 20% rule would only apply if my data, player, amp and phones were already the quality I can live with. If any of those is sub-par, I would not worry about the cable until those four parts of the audio chain are set.