Here's a variety of headphone amp suggestions for the
professional market (some are amps only, some are combos with DACs, in no particular order):
Lynx Hilo and Aurora(n) – excellent converters with equally excellent headphone sections. They're located near us and are nice people, too.
SPL Phonitor 2 – three inputs, and balanced in/out. Can be used as a line stage preamp.
RME ADI-2 PRO – a great 2-channel ADC and DAC, with analog inputs and some flexible DSP options.
Mytek Brooklyn and Manhattan series – great products, and they also do MQA.
Little Labs Monotor– made especially for pros, with various channel assignment possibilities for checking phase, etc.
Benchmark HPA2 and HPA4 – a great group of products for different use cases.
Rupert Neve RNHP – purpose-made for the pro world, with flexible routing and lots of headroom.
Anything from
Chord Electronics in the UK – their products are stellar and we often partner with them at trade shows.
Weiss DAC502 – Loaded with features and will soon have Audeze's LCD target curves from Roon and Reveal built-in to the firmware.
Linear Tube Audio MZ3 (or anything from their line) – Extremely accurate OTL tube amps that are hyper-detailed without being analytical.