Interesting OP. I am currently on the market for a portable rig and am looking for the flattest frequency response (without reducing quality) possible. I have heard the Clip+ is great (as the OP stated) but that the frequency response isn't flat. My research has not made me aware of any such thing (flat FR) thus far in regards to DAPs. I have wondered though, if having a good on-board equalizer (or an equalizer in any other capacity in regards to my portable rig... keep in mind I am learning and am a n00b) could compensate for the non-flat frequency response and ultimately achieve the same effect (I believe I may need pink noise or sine waves to do this...?).
I understand that the cans I choose can definitely affect my quest for a flat frequency response that results in sonic fidelity and am planning on consulting the headphones (full-size) forum on this matter but here I am most concerned about my DAP and how a DAC and portable amp could affect the frequency response. Any responses would be greatly appreciated! Please, help me out! Thanks for taking the time to read my n00b saturated post.