boring guy tells you why, always in a boring way:
the trebles can be different depending on the low pass filter, but given the extension of most IEMs, it should only rarely be audible at all.
the bass can be a mix of impedance damping, caps, and the impedance of the DAP changing the sound of the IEM(as soon as the impedance response isn't flat but still relatively low).
the A15 is 4ohm, the X1 is 2ohm and it's not so hard to find DAPs and sources below 1ohm(ideal).
here is an IEM measured with different impedance values as a source. no luck I didn't use 2 and 4, but 3 and 5ohm instead^_^, still you can all get the idea. the db variation marked at the bottom for each curve is at 100hz.
the yellow line is the impedance curve of the IEM to show the relation between impedance and frequency response. as soon as the impedance(yellow) of the IEM reaches around 30ohm( close to 9khz), the damping becomes ok for those values, and the frequency stops changing much. see how the source changes the bass? well it's 100% the impedance, the source is always my O2 I just add a resistor.
you can always avoid that by having an impedance ratio of at least 1/10 or lower, then all DAPs start sounding mostly the same(less than 1db variation).
in this example the bass goes down with a source(DAP or amp) of higher impedance, so it would be bassier on the X1(2ohm) than on a A15(4ohm), and even bassier on a sansa clip(1ohm) or a xduo X2(0.7ohm).
but it's 100% the IEM you guys are talking about when you describe the sound, not so much a coloration of the DAP. when I RMAA both, they have very little frequency response variations on their own. the changes are because of the IEM's impedance curve you have been using.
if you do the same with a Shure se846, the result will be opposite, and the A15 would be the bassier DAP, then the X1 with less bass, and a sansa clip with 1ohm impedance output would have even less bass.
I just gave you 2 IEMs that will say the opposite when you just describe what you hear in a X1 vs A15 testing. so how could you blame the DAP for 2 opposite changes induced by 2 IEMs? you can't, the guilt is on the IEM you're using for the test. with my 300ohm flat impedance etykid IEM(30$) all my DAPs sound petty much the same. because it's an IEM that won't get any hiss(not sensitive enough), and it also will not care about the DAP's impedance because with stable 300ohm, the damping factor is always great. so that's a third situation, almost no change at all between the 2 DAPs. ^_^
I explain this so that you guys avoid being so fast to label a DAP as being colored or boomy or whatever. almost all have a flat signature nowadays when plugged into a load(IEM/headphone) that isn't too low or too changing). what you described as the DAP's sound is always the sound of a combo DAP+IEM, usually not the sound of the DAP alone. so at least be clear as to what IEM/headphone you used when you describe your experience. it's mighty important if you want to make any sense!
i once loaded like 12,000 FLAC files to it (classical music fan here), seems fiio x1 removed song limit with a fw update if i'm correct
