Excellent device with the power to drive a surprising array of different headphones. Everything from IEMS, Hifiman XS, HD 660s, etc.
Huge improvement in quality for JH Lola compared with HiBy FC5. Plenty of power on high gain for Audeze LCD-2
But the iFi xDSD Gryphon I bought from Bloom is vastly superior
I bought the Questyle M15 Portable DAC and Amp after going through a suitable mourning period for the passing of my LG V40 smartphone (which had a great DAC) but LG abandoned that market. My new phone has a run-of-the-mill DAC and gives decent sound through a pair of 1More Quad-Driver IEMs. When I heard about the Questyle M15 and read the reviews, the price seemed right and I found it on b\oom audio's website.
After a few weeks listening to my music with this DAC I am very satisfied--especially considering its solid build and how easy it is to use. You get one option: low or high output. That's it for the button complement. I am using my IEMs on the low setting and there is plenty of output to the 3.5mm jack. Using the Poweramp Android app and flat EQ to play music, I used to set the volume around 60% to 65%. With the Questyle M15 the volume ends up usually no higher than 55%. These values are relative, I know, but they may be somewhat useful for you.
The M15 offers 3.5mm and 4.4mm balanced outputs. I don't yet have any headphones or IEMs I can use in balanced mode other than my Meze Elite's and the two cables are terminated in XLR and 1/4 inch connectors. My other cans--Sennheiser HD650 and Beyerdynamic DT1990 Pro--are terminated in 1/4 inch plugs. So I am now shopping for an IEM that works in balanced mode with a 4.4mm connector.
I can't really fault anything about the M15 sound; in fact, quite the opposite. Bass is full and deeper than produced by my phone's DAC (Motorola One 5g Ace). Mid-range is nicely done; just a bit forward and not harsh at all. Vocals are rendered as they should be with nice definition. Tracy Chapman, Diana Krall, and all the other audiophile standards sound very realistic and the M15 hangs their voices dead-center with a bit of a glow around them. The highs seem just a bit rolled-off, but that could also be the IEMs. Either way, the M15 produces the airy and delicate highs I'd hoped for with no bite or fatigue.
Soundstage seems much wider, though there is a bit of the "in your head" sense. I can't hear any height cues but maybe they'll show up when I upgrade the IEMs.
I know the DAC dongle/outboard processor market has several very good options--and I haven't heard any of them, so if another new processor beats the M15 in reviews or real life, then so be it. I am, however, very pleased with the Questyle M15 and enjoy my music more than before. Since it has no internal battery the M15 draws power from the phone, and battery life is excellent.
Shout out to b\oom audio for getting my new toy to me very quickly and in perfect condition! Great folks to do business with.