I used Hifiman Sundara + Fiio K5 pro. Recently ordered Hifiman Arya and since it needs more power I bought Topping DX7 pro. Everyone said it's so good. But I was really disappointed with it cause compared to Fiio the sound became cold, pinched, the instruments didn't sound natural to me, the music lost sense of scale. After many tests on different music genres, I ended up putting DX7 pro to the box and was thinking to return to the store, I thought something is broken. I started to read and realized that this is what people call "natural" sound. Why would anyone need "natural" if the sound can be "wow" and give goosebumps? But that's another topic haha.
I replaced stock op-amps(LME49720) with 2x Muses01 and 2x Alfa AS402 in the filter section. Cause I've heard Muses01 uses FETs and gives warmth and depth while keeping details and Alfa uses bipolar transistors(that works well combined with FETs) and gives clarity and good details. I am not really deep in the technical side but I decided to try. After the op-amps change DX7 pro is completely another level device. The sound changed not 1% or 2% percent but it's a full transformation to me. Everything is musical, got depth, instruments are natural and detailed. Now I enjoy it a lot.
Someone was mentioning Burson V6 here. I've also heard they are quite good and move the sound to the direction I like, but for some reason, they don't say that Bursons don't fit the size of the device and you'll not be able to close it.
One more thing I learned - under the radiators there are few more chips AD8397 which play the role of digital to analog converter. Replacing them can also help improve the sound and better predict the final result of other OP-amps influence.
So far conceptually I am really happy with what I got but maybe will experiment more in the future