Might be a little too late to chime in but here are mine.
Ideal DAP:
Size and weight
between Ipod and DX100
Battery
Playtime 6-8 hrs charging 2-3 hrs.
built in and last at least for 3 years. After that there's going to be a new one anyway.
UI and designs
A dedicated << O > >> up and down plus main menu button is a must. Simple layout, solid feel and responsive. Use a little part of high end material on the body at least the front of it for that high end look and feel. Nice soft blue LED light and a dedicated lock button.
The Ins and Outs I'll leave it to you.
Sound:
Dual DAC top of the line Sabre chip whichever sounds best. The sound should be neutral with a little warmth, excellent clarity but not sterile like DX90, good impact and bass body, superb imaging and 3D sound space. Works as stand alone DAC.
Amp section:
Just similar to X5 but better. I would suggest not to invest a lot on it unless you can do it for less. You can't please everybody no matter how good it is folks will still prefer to use their own favorite amp anyways.
Features:
- Wifi streaming from PC or MAC, plays third party app such as Spotify etc.
- Hardware quality 10 band EQ with 3 user own presets and pre amp level.
- Sound customization/adjust.
Different headphone has different freq problem such as lack of bass, lack of body, muddy, closed sound, too bright, and so on...
It will be helpful if you design a built in sound adjustment.
Create:
Bass ( >170Hz) increase or decrease (+/-) 3db to adjust bass respond.
Lower mids (150Hz-250Hz) +/- 3db to adjust body and weight
Upper mids (3Khz-5Khz) +/- 3 db instrument definition and attack
Highs ( above 10Khz) +/- 3 db for detail, clarity and air.
- Crossfeed: out of the head sound
- Virtual environment: natural, club, hall
Some people might think it's a joke but when done right it could add enjoyment especially for EDM and classical music. The effect should be subtle or less dramatic so it doesn't sound fake.
- Volume equalization: same level for all tracks.
- Night mode listening: increase low freq when listening on low volume.
- Freq spectrum extension: add high and low extension to a compress file ie. mp3, aac.
- Easy add/create to playlist
- Battery indicator: shows how many percent left
- Storage: at least 32 Gb on board and two micro slots
- Search function: find a specific file ( album,title artist or songs)
- Docking station: charging and nice to stare at.
- Touchscreen would be nice.
- Comes with protective case ie. leather, silicon or synthetic leather.
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Other than that please don't cut corner on looks. A lot of idiots buy things for looks and popularity instead of real quality. Also most people feel better when they purchase something expensive so ..price it right !