Headphones

Ok so, I did a test. I'm confused. I need your input to figure out what's happening. I mix and match 3 DAC/AMPs with 2 IEMs. The question I have is that, if DAC/AMPs are supposed to be transparent, aka outputting a flat FR, what else can affect the tonal balance in the chain?

Gear list:
HIFIMAN EF600(Low Gain NOS), Cayin RU6 NOS, FiiO BTR7
Truthear Hexa, Moondrop Aria 2

I tried my best keeping all the volume same, vocals especially. The EF600 has A LOT more bass than the other two, but also having a much clearer instrument separation and clarity, by a great margin. BTR7 has somewhat good bass while RU6 has the least. RU6 has the least clarity as well.

I know this may sound obvious, "Duh, DAC/AMPs are different." But I'm wondering the root cause of this, like is it because they output different non-flat FR? Or is it the output impedance that's affecting the sound? I specifically used two IEMs with different setups, single DD and 1DD+3BA, the results are exactly the same so IDK if it is impedance. I'm curious what's behind this. I know RU6 has very bad THD so a lack of clarity can be explained. BTR7 has better THD on paper than EF600 but the actual perceived clarity is audibly lacking. It could be IMD or something else but the drastic difference of tonal balance is beyond my understandings.

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