Heya,
Plugging headphones like these into a cellphone as a source is going to give you the experience you had. These headphones were designed with the intention of them being plugged into an amplifier beyond a cellphone. Not gobs of power like a speaker amplifier, but more than what an ultra low power cellphone generally is capable of (they're not designed to power these kinds of audio devices, more designed to simply give something to extremely sensitive IEMS mostly).
It also matters substantially the quality of the source material you play back. Sending some low resolution compressed media through them will reveal flaws in the source material. So be mindful of what you listen to in order to evaluate them.
The DT990PRO's are generally 250ohm but fairly sensitive. The AKG712's are supposed to be sensitive and are around 62ohm but they're notoriously not easy to drive from a low power source and they are really flat with no exaggerated mid-bass bloom so they are generally not a headphone for someone looking to walk around with a cellphone and listen, etc, unless you're in a quiet place and want a very flat response experience. The Fiio E10k will handle the Beyers fine. The Beyers have hot treble & exaggerated mid-bass. But ultimately if you don't like a "V" frequency response headphone, like many Beyers tend to be at the entry point (DT990, DT770),then it may be best to try something else. Depends if you want a more netural presentation or something "bassy." The Senn HD600 for example would be more neutral. A neutral Beyer is the DT880 and they are excellent. There are tons of headphones in the $200~300 range that are all excellent but have their own signatures. Lots of them do need a tiny amplifier, more than what a cellphone typically would provide, but it doesn't have to be expensive (an E10K like you pointed out will handle most of them just fine).
I've had all of these headphones, so if you're interested in anything, let me know, I can at least describe their signatures from experience.
Very best,
