I just received a Questyle CMA18, purchased through Amazon.
I fell in love with the Questyle M15i dongle-style DAC/amp and got the CMA18 hoping for an upgrade as well as the flexibility of Bluetooth. I really wanted to love it.
It worked marvelously with my Focal Utopia and Focal Stellia. However, I had a poor experience using it with more sensitive IEMs. I tried 3 different IEMs: the 64Audio U4s, 64Audio Volur, and Empire Ears Odin. All three were setup to use 4.4mm balanced. The noise floor was terrible, and they all exhibited a background hum and clicking sound during quieter musical passages as well as when nothing is playing.
Used with my conventional headphones, I had no problem driving both the Stellia and Utopia. There was no evidence of a noise floor problem with them; the sound was clean and to my ear neutral in nature and I was quite happy with that.
Another issue I didn't like is that I needed to set the iPhone at a very high volume before I could use CMA18 (both conventional and IEMs). With the M15i, the fixed output level matches the iPhone levels fairly closely, so it's very intuitive to use with the phone controls. With the CMA18, I have to crank the phone before the amp volume buttons can be used. As a consequence, I have to be cognizant of that volume setting and remember to set it back to a lower volume whenever I'm finished with the amp. Not a showstopper, but irritating.
Re: Bluetooth...it was very easy to pair, but the placement of the "function" button makes it far too easy to hit accidentally, which switches the input selection. So after an accidental press you have to cycle through all the selections to get back to BT. Reestablishing the Bluetooth connection is simple but it takes a moment, which is an irritation since it is so easy to change by accident (and happened frequently during my testing). Also, while the use of circuit board LEDs visible through the glass window is a Questyle signature, it does not serve the CMA18 well. It's quaint and inobtrusive on the M15i, but the CMA18 is far more complex and this interface is very awkward. At this price point and this level of complexity, they should have provided a more sophisticated display. I understand their commitment to their design language, but in this case it is way off base.
If I were using this only with conventional headphones, I might have been fine keeping this as a nice portable, but I started playing with portable amps mainly to support IEMs. For that application, the noise floor is a showstopper.
That said, I don't see anyone else complaining about a noise floor in this in the thread, so maybe I got a lemon?