If you haven't seen this thread, it may have some additional insight:Looking for a recommendation for a first time tube amp.
I've never even heard a tube amp but now that I have my third ZMF on the way I think it's time to dive into that pool.
I've settled on the following two devices because of their price point and small form factor:
Feliks Echo MK2
Woo Audio WA7 FireFlies Gen 3
I'd be running my Questyle CMA400i as the DAC for the Echo.
https://www.head-fi.org/threads/the-un-official-amp-thread-for-zmf-headphones.965309/