I tend to regard the M50 as the minimum for hi-fi listening with no apologies for limitations. But if you have to go cheaper than the usual $125 USD for the M50, the B&O Form2 with a simple bass boost gives a real hi-fi sound, and it should be available for about $100. The Beyer DTX-501P (similar to soundmagic P30) is good for $100, but needs a slight treble boost. The LSTN Fillmore with wood cups is somewhat colored, but still a good listen and good for $100. Below $100 are the Sennheiser PX series – not hi-fi by any means. There are several good IEMs below $100, and Apple Earpods with a Dirac or Accudio Pro player are very hi-fi, and cheap.
The FiiO E17 “Alpin” + AIAIAI TMA-1 or Pro700 Mk2 seem like a mix you’d recommend, but I’m concerned those cans will be uncomfortable for day-long use. Also I don’t know if that DAC/amp is the best for my style of music/headphones. Third, some of my techno tracks have stunning vocals, and I don’t want to miss out on those high notes while enjoying the bass.
I’m a former Mixer and DJ and I’d highly recommend the Beyer T1 for mixing. It is not absolutely flat, but it is an all around proper Audio Technicians Headphone. For flat response, I might eyeball the HD800 but the low end might not be the best if you are often mixing tracks with solid low ends. Flat might not always be the best option ( my opinion ). If you need to mix heavy bass tracks, it is better to have a headphone that responds better to low end EQ and is generally accepted as a more well rounded headphone.