Sorry, that's my first post here
If anyone still has an interest in modification of Tera by Russian enthusiast Symmcat, I can try to explain the difference in a short way.
1. The amount of lows was increased. For now, it has become really neutral in this aspect. Moreover, the control of lows has become better too. It can be obvious if you listen bands like Norma Jean, Chariot and so on. Better control made lows more detailed. That's the level of Hifiman HM-801 with custom amp.
2. Mids are not so sharp anymore. I did not notice any sibilance with using Alclair Reference, Etymotic ER-4S and Fischer Audio TBA-04 as my favourite IEM. Even though, the amount of details was increased significantly. However, mids are not as airy, as in stock Tera. They are more "brutal", but the separation of instrument stayed as amazing as it was in stock.
3. Highs are controlled better, than in stock device. Their amount was reduced. Symmcat is not a big fan of so bright sources.
Generally, Tera Symmcat Edition can be named as a little brother of a good-old Hifiman 801. Level of quality is the same. While 801 still has better bass texture, Tera SE is more clear in terms of highs.
If anyone has any other questions, I'm ready to answer. My collection of DAPs consisits of Hifiman 801SE, Ibasso DX100SE and Ihifi 960. However, as you can see, my English is so far from being perfect