it is a niche market. there are also too many headphone amp companies now, because it costs little to start making them and doesn't require too much skill. it would be very hard to exceed $250k/year revenue per model for a >$500 headphone amp product. i think STAX revenue is only a few million $ per year? considering Beats has a 70% marketshare of the >$100 headphones, giving STAX a tiny fraction of a percent is about right
this is all fine for a 1 man show to make a bunch of money, but dont expect to see headphone amp companies with corporate HQs with a receptionist and a marketing team and an engineering team anytime soon. though i would bet on Schiit being the first, their $99/each stack has gone well beyond Head-Fi by now