HE-400 + O2 + ODAC has been my go-to setup for going on 2 years.
I have also used my HE-400s with a Realtek Chipset (~2010 vintage can't remember the exact version but it wasn't their best) and the more recent 892 chipset. I have also tried a Yamaha reciever, 2 Nokia Phones, 1 Asus Tablet and so on.
In addition to listening to all those devices standalone, I've also used them as the input for the O2.
In addition to that, I've plugged every headphone I had around into the O2 to "see what happens".
So, I am your HE-400 authority :wink_face:
Disclosure - I buy amps for the POWER I think I need, not for the SQ they supposedly add / take away from connected products. I just like high power, low distortion, connectivity etc. In most cases the rest is marketing IMHO. If you REALLY want to change your sound, change your speakers. That is my rule #1.
In short, the O2 is just a great amp PERIOD. It has had more than enough power under all listening conditions to drive the HE-400s to levels I can't stand. IF you are using a 1V source (which includes everything I listed aside from the ODAC), you would be wise to keep the high gain (6.5x) handy. For HDR recordings, the added headroom can be important. The battery life is excellent (routinely lasting for hours), and the thing has the blackest background I've ever owned, from any amp (even component amps I own for regular speakers). It was actually strange at first, until I got used to it.
Now, the ODAC added what I felt is a "superior" SQ after intimate listening (somehow just a little punchier and more dynamic and equally black). BUT the realtek chipsets and even the phones / tablets I own actually sound darn good routed through the O2 (The 892 and the Asus). Both presented a similar sense of depth and were smooth. I think the Sabre DAC might actually be "more aggressive" (not in a bad sense but in a sense). Then again other times the DACs all sound about the same to me. So trust me when I say that the DAC is probably only the last 5-10% of SQ I was able to squeeze out of the HE-400s and the first place I would recommend you save your money for now.
"Dry/Analytical" are ridiculous terms to me. The O2 is basically whatever your headphones / recordings are (and guess what, maybe your phones / recordings s***!). I bought mine from JDS labs, and John Seaber was quick to ship, quick to answer questions / etc. What is more, the price of the O2 is now lower, and you can customize it to your liking (rear power jacks, 1/4 inch front jack etc.). Certainly I would have taken advantage of those things if I were doing it all over.
I've not owned any Schiit Gear, but the Magni would be on my list as a second choice. Maybe they listened to my griping about how the first Magni had fixed gain (its major fault IMO - 5X with 1 watt!). Then again I don't like the RCA inputs either. I generally have had my Headphones connected to modern devices like computers and the aforementioned, which have forever been using 3.5mm inputs/outputs. Just makes sense.
So, I would buy an O2 again without hesitation. I just don't know of many other choices in your price range that combine the features / SQ.
Hope that helps.
