I've been reading so many replies and reviews how the O2, Magni and Vali sound the same. After weeks of listening to all of them, (yes I have them all) they are all not exactly the same . I've been using the same music with the HE-400 and all else in the system is the same and have only changed the amps. Started with O2, then went to Magni. Yes the Magni and O2 sound very close. The Magni does sound a tad processed and thin compared to O2 but the Magni does have more authority as it is more powerful in comparison even though they all seem to struggle reproducing the source with the HE-400.
Then I went to the Vali. The Vali sounds nothing like the others. It's slightly more smoother and warmer, more of a natural sound but ever so slightly, and more realistic sounding. I was enjoying it very much for weeks. Then I decided to get the Asgard 2. Why? I read so much about it I just had to try it. The Vali also seem to struggle reproducing the source which makes sense as it's less powerful of the bunch.
I have to say the Asgard 2 is is not like any of the others. Why have some said there is no difference? I'm suspecting what is happening is that people are not using high resolution (or not high enough resolution) tracks or can distinguish the difference OR they just don't hear the difference due to their hearing biology. As a side note: I also did an electronic spectrum hearing test to verify my hearing at different frequencies to eliminate that factor, and I passed 100%.
The Asgard 2 with the HE-400 sound way more detailed, realistic, bassier, deeper, smoother and wider is the best way to explain it with all else in the system being the same.
So for anyone that is thinking about the Asgard 2 with the HE-400, YES it's worth it. The others are a good start, but You WILL hear a difference with respect to what was mentioned above about headphones, music quality rate and biology with better amps. This combo, sounds like an actual amp, it sounds "real" while the others mentioned sound like they're struggling to making an effort to process the sound.
I hope this helps anyone who maybe interested in how the HE-400 responds to different amps.