So rather than iBasso DC04 Pro I've got Kuang Pai Plus 4. What an upgrade! Sounds better in pretty much every aspect. Has a resolution BTR15 doesn't have and an instrument separation and imaging ahead of competition, in areas where the other dongles congest. Transparency among the best I've heard on a portable device. Treble extension as good as my Antelope Audio interface (unlike BTR15) with spectacular detail and refinement. Bass is tighter than BTR15, superior definition/texture. Here you don't just feel it, but hear every note, texture, harmonics, crystal clear slap bass, the sound of fingers running through the bass strings, etc. The lowermost details.
Taiwanese brand, just one review here on Head-Fi.
Sounds almost as good as Topping DX5 (400€).
Real upgrade over sub 200€ dongles and even above and beyond.
It's a very sophisticated equipment, in some areas in the same league of Topping. Micro-detailing and timbre are exceptional. The sound of leaves, birds and other nature elements sound real. Percussive instruments, flutes and stuff like that, sound amazing. Let's say that BTR15 in comparison sounds a bit dry, dull, lifeless, not revealing those details with Plus4 accuracy and realism. Full of life (superior detail retrieval, separation and positioning of sounds in the 3D space).
6 filters that also contribute for that with 6 different sound signatures. And all about noise reduction in every way possible, including a dual motherboard.
BTR15 still has a space in my heart, sounds fantastic for 120€, it's very engaging, but Kuang Pai works better as a portable audio interface on the go, due to its sonic superiority and flexibility with a more powerful amp, capable of driving headphones like HD800 with ease. Great discovery, I couldn't be more impressed. Price was 155€ on AE.
I didn't give iBasso DC04 Pro a chance, but I'm not going to after listening to Plus4. It's all I need.