I would be using it with an HE400. I find the HE400 is pretty bright with the Magni to the point where it needs EQ. I'm looking for something that could tame the treble on the 400 possibly allowing me to use them without EQ but I'm fairly nervous about spending $450 on an amp (I only spent $300 on the headphones). Everyone says that the Lyr and 400 pair well but I just REALLY want to be sure.
Unsure if you are in USA, but cheap shipping and Schiit return policy of a new Lyr certainly lessens the risk.
not having HE-500 (skipped HE-400 for the treble signature) nor Magni,
but having Bifrost Uber + Lyr (with myriad of tubes) now and HE-500, Vali on a different set up and sold an Asgard2:
all I'll say is Lyr can definitely be made "warmer" to sooth your HE-400, especially if you pick some warm tubes.
unsure what DAC you have, but the customizability of the Lyr with some Orange Globe or Mullard or <insert warm tube of the month here> tubes
can dial in the sound signature you prefer.
THAT (IMHO) is the real benefit of the Lyr.
You'll need to be willing to jump in to some tube research and add on purchases, but in the end- it's more likely to be "perfect"
than trying a Magni and/or 10 different solid state amps before reaching similar results.
Summary:
in expensive try n buy could be the Vali tube amp?
more expensive option:
the Lyr could be perfect with HE-400 for your ears, IF you are willing to try some differing tubes and perhaps augment your DAC.
the Lyr (plus Ediswan, RTC, Siemens tubes) with my HE-500 (and Concero HD) are pretty much my end game.