Got my weekly Head-fi email and noticed interest in Stax SR-X9000 & others.
OK. OP heard Sony ZR1 and Utopia and no like. Those are already aces for their time. Trade publications placed Utopia above HD800, etc. ofc, this was before Abyss and Susvara.
To cut this short, I own most of these and many are rotated less now, as new models come into play.
In terms of amps, you actually can get excellent use with a $900 portable amp like this one with the SR-X9000. Great when you're on a boat, RV, out in the woods.
To get maximum use out of the SR-X9000, you'll want some tube amp. I had many, Blue Hawaii, Cary, etc., now I use the LTA Z10e. But others will work too.
Really depends on your music of choice. You want out of your head, pictures at an exhibition sound scape or pounding R&B. That will dictate which amp.
You mentioned Susvara. I love this with the Sony portable DAC/Player. On a boat. For my vocals and ensemble Jazz. This combo cannot be matched, even with full amps at home. The Susvara is favorite on the non Stax side. (The Abyss is a great alternative, but 11 out of 10 find it uncomfortable)
For DACs, I continue using a Chord DAVE with the M Scaler or the Nagra DAC with recent upgrades. DAVE is super for higher SPL music while the wider, more delicate variety shines with the Nagra.
See if you can sit somewhere and audition a few combinations. Good luck and trust your ears.