I was targeting something that “sounds good” to me, that’s comfortable on my head (not too heavy, not overly clampy) and that looks nice. My baselines were the HD58X, LCD2C and to a lesser extend the DT 1990. I’m shooting for something that
- Sounds natural with a neutral to neutral-bright signature
- Has decent bass presence but more in the audiophile sense, meaning good sub-bass extension and punch, but without sounding bloated
- A well balanced mid-range presentation, with clear vocals and lead instruments that fit into the mix rather than standing out from it
- Reasonably energetic but non-fatiguing, which for me primarily means that it’s got to have decent upper midrange presence but it’s better for the upper mids to be a tad recessed rather than a tad overemphasized
- Clear and sparkly treble without becoming sharp
I’ve come a long way to wards hitting these goals. The main area where I’m still tweaking is the treble, which is already less sharp than the DT 1990 but which I’d like to tone down even further if I can without sacrificing detail.
I’m using a 50mm moving coil dynamic driver with a composite paper/plastic cone (AFAIK it’s the same driver as in the Aurorus Borealis). It’s been really interesting playing around with the damping. The driver makes the sound, but the bulk of the tuning is actually in what goes around it. Just the bare driver with random pads and no custom damping sounds terrible, but with the right pads and damping things really come together.
Measurements don’t tell the whole story, but here’s a comparison of the current HH1 against an early version, along with the DT 1990 and LCD2C. This is all on a MiniDSP E.A.R.S. HEQ compensated, smoothed using psychoacoustic smoothing.
The DT 1990, LCD2C and current HH1 all look like serious headphones, while the early experiment HH1 looks, well, odd 
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The three serious headphones all have pretty good bass and midrange tonal balance up to about 3 KHz, with the DT 1990 standing out as a bit warmer, the LCD2C a bit shouty around 1KHz, and the HH1 a bit laid back in between 2-3 KHz. In the treble, the LCD2C is the dark one, the DT 1990 the bright one, and the HH1 lands in between, leaning towards bright.
As much as the tuning has been a fun challenge, the most time consuming part was actually the physical/mechanical design, especially w.r.t. to making it reproducible for making more of these.
