Yeah even if you don't like O2 "neutrality/organic-ness" and prefer tube roll off, you can be sure O2 has enough juice for the HE-400 to shine.
The Schiit might be able to compete (haven't heard it, aesthetically looks nice and price seems like a strength) but O2 is the 'tried and true.'
And don't forget, it is useful to have some gain switching. The Schiit seems to be a good product, but until I see some measured specs I might shy away from it. The O2 is pretty feature-minimal, but I reckon it is easier to match to different sources / High Dynamic Range recordings.