is it common to combine high level DAPS, for example Fiio, Astel &kern Sony with other DACs, or even with a headphone amplifier?
Well, there are much more experienced users in the forum here. I mean, what i will write is from my point of view on things.
You can combine everything, its not a problem. The main purpose of combining is the lack of power from the Dap s for powering high impedance headphones. Full sized headphones usually need a separate amplifier for getting their full potential. Lately the manufacturers are trying to solve this "problem" with the portable gear by.... if you look another Fiio players for example, X7, X7mk II, you will see they have amplifier modules you can buy as well. Different and more powerful ones. Other companies also have switchable amplifier modules for their Dap s. This is for to eliminate the need of separated amplifier device.
Regarding the devices and the sound, the difference in how you hear the music and its coloration comes mostly from the amplifier and the amplifying circuit inside. Dac chips themselves mostly sound the same, the difference between the dac chips is minimal. You can try switching the filters on the M9. Sharp roll off, slow roll off and what were the others.... so thats about the difference that you will hear between dac chips from different manufacturers. There is, but its minimal. Much bigger impact comes from the amplifier circuit used. In general slow roll off filter is how the AKM dacs sound, while Texas instruments, ESS are more like the sharp roll off one. On the M9 sharp roll off will sound closest to E18.
For example if you want to compare only the dac chips, on the E18 need to be used the bottom coaxial output. Then the amplifier circuit of the E18 is bypassed (that is equal to using the M9 with Fiio OTG cable like you did). Not that you need to do that, just if someday curious, you can try. But then you will need another device to receive signal from the E18 coaxial output.
The E18 sound is little bit more bright than natural, the higher frequencies receive somewhat +1 2 db boost. This is from its amplifying circuit inside. Another amplifier will have something different. Meybe more bass for example. And this is why the manufacturers have more than just one amplifier module for their dap s for example. And this is why they say it changes the sound color little bit. What will please a person more is difficult to be said, this is why most people will say you need to hear the things yourself before buying. That goes for headphones and all kind of audio devices.
If you want to read more about how sound is described, look at this topic:
https://www.head-fi.org/threads/describing-sound-a-glossary.220770/#post-2673350
How old the device is doesnt matter that much. Generally is believed the new stuff is better, because all manufacturers are trying to improve their devices and release new ones that are better. But because different people like different sound....there is no guarantee that new device will please all. You can find people with old Sony walkmans that just dont want change it with anything new.
You can also use equaliser to change the sound as you like. Overall there are many reasons on why you would want to combine devices, but in general with the portable dap s is the lack of power for full sized headphones.
Most difference in sound you will hear with the different headphones however. You can try one pair, then another with the same amplifier and DAC, and be completely amazed how different the sound is. So because this, i would say headphones are most important, then next most important is the amplifier ( it will give you different coloration and more power usually sounds better ) and the last with the minimal difference is the Dac used.
Hope this helps somewhat.
