I've been considering K5 and A5, but I'm not entirely sure if they have enough power to drive the headphones to its full potential (or pretty close). What would you say about Magni 2U? Or would you recommend me going with tube amp such as Vali 2 and Valhalla 2?
I've had a Bifrost4490/Asgard 2, now I use them plugged directly into a FiiO X5 3rd Gen.
If you listen to music at normal levels, you don't need much power for HD800. All of the amps mentioned above have enough power with extra room.
The full potential thing is a tricky concept, you might have to try something like a Schiit Yggdrassil/Ragnarok or Auralic Vega/Taurus mkII or a Chord Dave setup trying to find out where's the full potential to your ears. The "full potential" a tricky concept because at the end of the day, preference is a subjective matter and it's also influenced by many variables such as listening levels and enviornment, the recordings used to test, personal characteristics and so on... You will hear slight differences between amps and dacs, but they are relatively subtle in the big scheme of things.
All the mentioned amps can drive the HD800 properly, that said you might enjoy some of them just a tad more than the others.
I don't have much experience with Magni 2U in particular but it should work quite well.
I've owned the original Magni and also the Asgard 2 in the past, both worked well with HD800.
Asgard 2 sounded a tad more roomy and organic to my ears, while the Magni was a little bit too splashy in the treble region but still nice in terms of overall SQ.
As far as I recall Magni 2U is not as splashy in the treble as the original Magni. Still different than the Asgard 2.
Tube amps have pros and cons, they are typically less stable than SS and technically inferior in many regards.
If you don't care, then Vali 2 is a nice affordable pairing, and it'll surely sound good.