For a full in-depth review, see Delitia's review on head-fi at: https://www.head-fi.org/showcase/cayin-c9-totl-tube-solid-state-portable-amplifier.24966/reviews
The build quality is very good. It has great soundstage and warm bass
After a long period of thinking about the C9 and reading reviews on HEAD-FI by owners and responses by the makers, I jumped and made the investment. I had considered a desktop alternative, the Bryston BHA-1, but am thankful I went this direction. My overall impression is that all across the frequency response, there is better detail presentation, blacker background, wider soundstage, better macro-dynamics. The surprising thing to me is how much the C9 elevates LOWER-Fi IEMs. My Ikko OH-10s have never sounded so good!
A number of reviewers have noted that the C9 also elevates DAPs that have good DACs, but maybe are lower tier because of the quality of their amplifier. The Cayin N6ii & HiBy R62020 are examples. Higher tier DAPs put more into the amplifier section. It may be possible to pair the C9 with these lower tier DAPs and the resulting sound quality may be similar to the Cayin N8 (which also has 1 Korg Nu-tube); however, as a HiBy R8 owner, I found that using the C9 PRE-AMP setting, I can take full advantage of all the excellent features of the R8, especially the turbo feature. Using the C9 as a power amp in this way, makes use of the features of both devices to their fullest. My favorite mode is Korg Nu-Tubes + AB mode ...amazing detail AND impact. Listening the Thieaudio Monarch Mk2 IEMs via 4.4 mm output, using R8 as input with Turbo on, bass is detailed, fast, impactful, tight. Mids are appropriately forward and clear, treble likewise detailed, shimmering, never harsh. Soundstage is much more 3-dimensional than on the R8 alone. Everything is crisp, clear, impactful, yet not cold or dry at all. BTW, no hiss that I can discern.
If you are looking for a TOTL end game for your system, and have a range of IEMs across the price spectrum, the C9 is your go-to. Be sure to get the C9 external battery charger and an extra set of Sony Murata 18650 VTC6 batteries, so you can keep a charged set ready to swap out and keep the music flowing.
Andrew's service (and that of the Cayin team backing him up) was epic- I have never known a comparable vendor elsewhere that showed this level of responsiveness, willingness to help and commitment to make sure that my product experience is nothing other than excellent. You will not be disappointed.
I have a very bad habit of over-buying headphones and under-buying amplifiers. The C9 is a huge improvement for me. My wife was a bit of a skeptic until I let her listen. She summed it up: "the sound is more natural and has more energy!"
I now use my DAP when I'm on the move and then connect with the C9 when I'm at my desk or recliner and am ready to focus on the detail of the sound. This fits my lifestyle very well. Thank you!
I currently own the Cayin Nii6 Master DAP in tandem ~w the Cayin C-9 AMP, your world is rocking - The C-9 is the prettiest and most flexible portable amp out there to date .. 4 Modes, 2 Timbre (Tube and Trans), 4.4mm in/out, it's a thing of beauty. Add MusicTeck's (Andrew's) superb experience in doing business and it's Win, Win all the way.