Did you properly volume match? Most people don't. I did, and measured the output as well. The difference was in volume - nothing else. And with balanced vs SE IEMs, which I could rapidly A/B, the only difference turned out to be how loud they were. And of course we hear louder as:
- more vivid
- better separation
- better bass or treble
Etc
To back up what I found, I measured the frequency response (it's in the review) using the harder to drive MEE P1. If the output was "better bass and treble" it would show up on the freq response. There is no difference. And the reason they sound the same is that you are using either one or two OPA1622 (same chipset).
It's really easy to think you are hearing something when the playing field isn't equally matched. Human nature kicking in. It's why I always try to avoid potential bias when I review.