M15: the musician is the instrument
The M15 has arrived and so it's time for me to let you know what I think. When the M15 came out, maybe many like me thought... the usual dongle with the ESS (yet another) with a low power on paper and an "exaggerated" cost. Yet I should have realized something was wrong, after all I have QP2R and it's anything but the usual dap. The M15 looks like a minimal dap, no display, no volume control, no selectable filters, only the gain switch, basically nothing at all... instead it has it all. Some criticize it for the "classic" background noise of the Questyle Current Mode Amp, with my Mest MKII 12.3 Ohm no problem and perhaps it will also have it with iems that approach 0 Ohm but this is at most a problem of the iem and a feature of the CMA Questyle which, among other things, makes this dongle really special. The ESS chip has been completely emptied of all its "normal" characteristics and filled with the musical wisdom of Questyle engineers, we can say that it is the ESS chip with the furthest sound from an ESS chip. There are three characteristics that immediately catch the ear: Refinement, Musicality, Dynamics (it is no coincidence that I wrote them all in capital letters). Precise as a delta-sigma with a musicality that brings it close (but not too much) to a R2R with a simply reference dynamics (that's what the CMA Questyle was for, not to make 0.5 Ohm iems hiss). Defined, powerful and taut bass, reference mids and highs that are never offensive but very extended. It does not have an L, U, V or W sound, all frequencies are of the same level, it does not favor any frequency, it enhances them all. Once listened to and tried with the IEMs, due to the power it has, you will have the impression of having a pocket nuclear power plant... be careful!!! No kidding!!! One reviewer compared it to an iBasso DX300 and couldn't figure out which sounded better...now I know he wasn't kidding. Using an ESS chip doesn't mean playing like an ESS, having a miniaturized amplifier doesn't mean it's not really powerful and above all extremely dynamic, not being a dap doesn't mean you can't play at those levels (in some cases even much better). Costing 250 euros for a dongle doesn't mean it's expensive. If there is one dongle that can truly complement the RU6 it is the M15. Congratulations to Questyle musicians for creating this instrument!
Based on the specs the Questyle M15 seems not that powerful compared to other dongles.
But apparently the Questyle M15 is a current-biased amplifier, not voltage-based, so their amplification technology is very different from the usual source specs, they call it "Current Mode Amplification". So the M15 maximizes the current but not the voltage. Apparently these current-biased amplifiers only deliver juice when needed, and just enough to avoid clipping of the signal.
It seems to drive some of my harder to drive gear decently on high gain/4.4mm (eg Final E5000), but the RU7 is definitely more powerful with regards to driving power.