1. What Amp/dac should I get for use with a smartphone between 2-5k?
2. Whats makes about sonata HD pro great? a lot of people suggesting it here.
1. If your phone has USB C, and you have UAPP, I would highly recommend these. These can drive my HE400i very well, though I have to trick it to high gain mode by first putting in a 3.5mm extension without anything plugged into it. But since the Deva is only 18ohm compared to HE400i's 32ohm, you may not need the high gain mode actually.
2. Tempotec is the OEM of Hidizs, and the Sonata HD Pro is literally the same product as the Hidizs S8 (twice the price of this) but in an all aluminum enclosure. Reviews have it measured very well, making the dragonfly red and cobalt almost redundant, at a much lesser price. Check out head
audiosciencereview's review for the same. Even personally speaking, I find it excellent - both the DAC and amp bits are really good, and I don't think I want any more power for now.
Also congrats on the Deva, and enjoy the pair! I read they have a bit more soundstage and better bass roll off than the HE400i, so I'm sure it'll sound even more fantastic out of these! Without these, my pair used to feel hollow but now it basically sings. I kinda regret not going for the Deva for 3k more, but then again my initial budget was 8k for headphones so I was already over that haha.
If possible, make sure to eq these - the HE400i sound even better with oratory's preset to me, though personally I have kept the mids (filter 4) at only +3 instead of +6.8 since I prefer that.
Anyhow, enjoy your pair!