Been seeing a lot of high praise recently for the new, updated Chord Mojo.

I have the Chord Mojo 2.  It is very portable, and if you don’t have to have tubes, sounds very good.  I connect it to my MacBook Air for streaming with an $89 Audio Quest USB C to C Cinnamon cable.  Sounds like it might fit your portability needs well.  It is even smaller than you think!  About half the size of my iPhone XS.

Agreed with both Lon and effluviography - ZMF headphones and tubes are a synergistic pairing, and you'll get more out of your headphones with more power. The soundstage you're after only opens up when there's significant power headroom.

At your budget and with the semi-portable requirement, you'll be challenged to maximize the sound from the Atriums (mostly because portability forces such extensive tradeoffs).

Some ideas:
For all in one, the Mojo 2 is a great bet, or even the original Mojo which can be had used now for $200-250. If you want to stretch your budget, $1500 will get you a used Chord Hugo 2 which dramatically upgrades both DAC and Amp from the Mojo, but sounds sterile to my tube loving ears.

If you can stomach a bit more size, but can still be moved room to room:
Used Schiit Bifrost 1 Multibit ($350)
Cavalli Liquid Platinum ($400 used, $600 new)
with a couple bucks left over for a nice pair of 6922s.

Other option for a bit more size:
Used Chord Mojo 1. It can be set to DAC only mode ($250)
$750 tube amp. Used CSP2, Feliks Echo, LP etc

If you want an All-in-one with great WAF, Woo Audio Fireflies looks great and sounds pretty good. To my ears, not as good as the options above, but it's more portable and again has excellent WAF.
$900 new
$600 used
https://wooaudio.com/amplifiers/wa7

Hope that helps,
Richard