An iPad's not a phone. And the $9 Apple USB -> 3.5mm dongle measures as well as much more expensive DACs. If you've got a phone, and it provides enough power for the headphones, why go spending hundreds of dollars more? That's not really what this site is about.
remind me please the output power of this dongle?
when you have an amp circuit, that can do max 1W, it will behave much better delivering 0.5W (SINAD etc.) vs the amp, that can do max 0.1W and you need this 0.1W always - you will simply push that one to the limit, and we all know: at the limit, it will behave... at least mediocre