There appear to be two basic grades of 'stick' or 'dongle' dacs, 1V output and 2V output, depending on the module they have used internally. The low voltage versions - the CX31993, NICE-HCK and JM20 are really intended to drive IEM's, and low impedance IEM's at that. For higher impedance headphones, I recommend get the 2V versions which are the 'MAX' variants I tested, those typically have an additional audio op-amp buffer driving the output rather than relying on the dac chip's built-in audio amp; typically an SGM8262 from SG micro (also used in the Fiio K11 R2R). The exception to the rule being the TRN Black Pearl which contains 2 x CS43131 dac chips, so that each chip only has to drive one headphone drive unit (one CS43131 chip per side).
In any case, when driving better quality headphones like the HD650 and DT990, I recommend using a proper headphone amplifier to get the best possible sound quality. Even the cheap Douk Audio U3 from aliexpress can do a reasonable job, for very little money. And in my case I have the Firestone Audio 'cute beyond' class-A, which is very nice, and also the Canford 'bbc' broadcast quality headphone amp, that's probably a bit hard to find outside the UK. And if you have $3000 Focal headphones... I guess you already have a good amp to use with them, and you can ignore everything I say!
Of course you can get much better headphone amps than the two above. If you want something really good, get the Musical Fidelity X-cans V3...
Or if you can afford it... the Graham Slee SOLO is of course very nice, really, you can't get much better than this no matter how much you spend. Well, something even more expensive might measure better, but I will bet 10 dollars that you won't be able to hear it!
Graham is one of the world's top analog audio designers.
Of course with the exception of the Firestone which comes from Taiwan these are all British designs.. I'm sure there are very good Japanese and US designs as well, I remember Audio Technica AT-HA5000 is another good class-A headphone amp, using the famous Hitachi power mosfets. This one, I wish I could get some day...
Remember, whichever amp you have, one secret is to use a good quality linear regulated power supply, not a switch-mode wall-wart. A good linear PSU will give you a big upgrade to the sound quality, even with a cheap amplifier.
Of course anyone can spend $10000 and get a good sound, that's the easy way. It's more interesting to think "how can I get good sound from $300".
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